Drowned causes crashes (ticking entity)

by analyzing decompiled sources of 18w11a, I managed to track down that afn and afj classes both descend from afk class, which happened to map to net/minecraft/pathfinding/PathNavigate in 1.12 and afn and afj to zb and zf in 1.12 source, which is net/minecraft/pathfinding/PathNavigateGround and net/minecraft/pathfinding/PathNavigateSwimmer. Seems like zombies AI tasks are wrongly mapped between these two subclasses.
The error happens in the equivalent class of net/minecraft/entity/ai/EntityAIDoorInteract (wu in 1.12)
based on the lines and code structure, this happens when Drowned tries exexute this statment (attached screenshot)

Same problem here. I just switched my server to peaceful and it works now. Hope mojang fix it asap.

Can confirm on Win7 x64 18w11a

Server has crashed twice.

Here's one of my crash reports

--
---- Minecraft Crash Report ----
// I just don't know what went wrong :(

Time: 3/15/18 2:56 PM
Description: Ticking entity

java.lang.ClassCastException: afn cannot be cast to afj
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at rp.w(SourceFile:354)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Unknown Source)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Server thread
Stacktrace:
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)

-- Entity being ticked --
Details:
	Entity Type: minecraft:drowned (ajg)
	Entity ID: 29492
	Entity Name: Drowned
	Entity's Exact location: -12541.50, 41.98, 6687.40
	Entity's Block location: World: (-12542,41,6687), Chunk: (at 2,2,15 in -784,417; contains blocks -12544,0,6672 to -12529,255,6687), Region: (-25,13; contains chunks -800,416 to -769,447, blocks -12800,0,6656 to -12289,255,7167)
	Entity's Momentum: 0.00, -0.10, -0.03
	Entity's Passengers: []
	Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)

-- Affected level --
Details:
	Level name: world
	All players: 2 total; [sg['Matrix_Dev'/10887, l='world', x=-12459.31, y=71.00, z=6659.39], sg['Lighttt_'/12855, l='world', x=-12474.19, y=71.00, z=6654.64]]
	Chunk stats: ServerChunkCache: 588 Drop: 0
	Level seed: 1505535
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (192,64,-12), Chunk: (at 0,4,4 in 12,-1; contains blocks 192,0,-16 to 207,255,-1), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
	Level time: 2848325 game time, 3160263 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 127503 (now: false), thunder time: 91932 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at rp.w(SourceFile:354)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
	Minecraft Version: 18w11a
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.8.0_161, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 303977672 bytes (289 MB) / 624951296 bytes (596 MB) up to 3808428032 bytes (3632 MB)
	JVM Flags: 0 total; 
	Profiler Position: N/A (disabled)
	Player Count: 2 / 100; [sg['Matrix_Dev'/10887, l='world', x=-12459.31, y=71.00, z=6659.39], sg['Lighttt_'/12855, l='world', x=-12474.19, y=71.00, z=6654.64]]
	Data Packs: vanilla, file/veritas0923 (incompatible)
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)

If you encounter this problem, you can fix it by using a NBT editor (e.g. NBTExplorer) and set your gamemode to peaceful (level.dat → Data → Difficulty → set to 0). However, it can happen again after setting your difficulty such that Drowneds will spawn.

Another Drowned crash Hope it helps

and incase it makes a difference i was playing with bubble streams and upside down stares in a bubble stream

OpenJDK 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
[12:27:08] [Client thread/INFO]: Setting user: MEMEME
[12:27:09] [Client thread/WARN]: Skipping bad option: lastServer:
[12:27:09] [Client thread/INFO]: LWJGL Version: 3.1.2 build 29
[12:27:09] [Client thread/INFO]: Reloading ResourceManager: Default
[12:27:10] [Sound Library Loader/INFO]: Starting up SoundSystem...
[12:27:10] [Thread-2/INFO]: Initializing No Sound
[12:27:10] [Thread-2/INFO]: (Silent Mode)
[12:27:10] [Thread-2/INFO]: OpenAL initialized.
[12:27:10] [Sound Library Loader/INFO]: Preloading sound minecraft:sounds/ambient/underwater/underwater_ambience.ogg
[12:27:10] [Sound Library Loader/INFO]: Sound engine started
[12:27:11] [Client thread/INFO]: Created: 1024x512 textures-atlas
[12:27:12] [Client thread/INFO]: Narrator library successfully loaded
[12:27:12] [Client thread/ERROR]: Realms module missing
[12:27:18] [Client thread/WARN]: Ambiguity between arguments [teleport, destination] and [teleport, targets] with inputs: [Player, 0123, @e, dd12be42-52a9-4a91-a8a1-11c01849e498]
[12:27:18] [Client thread/WARN]: Ambiguity between arguments [teleport, location] and [teleport, destination] with inputs: [0.1 -0.5 .9, 0 0 0]
[12:27:18] [Client thread/WARN]: Ambiguity between arguments [teleport, location] and [teleport, targets] with inputs: [0.1 -0.5 .9, 0 0 0]
[12:27:18] [Client thread/WARN]: Ambiguity between arguments [teleport, targets] and [teleport, destination] with inputs: [Player, 0123, dd12be42-52a9-4a91-a8a1-11c01849e498]
[12:27:18] [Client thread/WARN]: Ambiguity between arguments [teleport, targets, location] and [teleport, targets, destination] with inputs: [0.1 -0.5 .9, 0 0 0]
[12:27:18] [Client thread/INFO]: Loaded 0 recipes
[12:27:18] [Client thread/INFO]: Loaded 0 advancements
[12:27:18] [Server thread/INFO]: Starting integrated minecraft server version 18w11a
[12:27:18] [Server thread/INFO]: Generating keypair
[12:27:19] [Server thread/INFO]: Reloading ResourceManager: Default
[12:27:19] [Server thread/INFO]: Loaded 519 recipes
[12:27:19] [Server thread/INFO]: Loaded 562 advancements
[12:27:19] [Server thread/INFO]: Preparing start region for level 0
[12:27:19] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:19] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:19] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Preparing spawn area: 0%
[12:27:20] [Server thread/INFO]: Changing view distance to 12, from 10
[12:27:22] [Server thread/INFO]: MEMEME[local:E:9cabb5c3] logged in with entity id 128 at (1501.6205663607718, 60.680039828981215, 829.0207043670652)
[12:27:22] [Server thread/INFO]: MEMEME joined the game
[12:27:23] [Client thread/INFO]: Loaded 204 advancements
[12:27:24] [Client thread/WARN]: Unable to play empty soundEvent: minecraft:entity.cod.ambient
[12:27:24] [Server thread/ERROR]: Encountered an unexpected exception
h: Ticking entity
	at net.minecraft.server.MinecraftServer.w(SourceFile:706) ~[18w11a.jar:?]
	at net.minecraft.server.MinecraftServer.v(SourceFile:627) ~[18w11a.jar:?]
	at czd.v(SourceFile:155) ~[18w11a.jar:?]
	at net.minecraft.server.MinecraftServer.run(SourceFile:532) [18w11a.jar:?]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151]
Caused by: java.lang.ClassCastException: afn cannot be cast to afj
	at adb.a(SourceFile:41) ~[18w11a.jar:?]
	at acz.a(SourceFile:22) ~[18w11a.jar:?]
	at adl.a(SourceFile:94) ~[18w11a.jar:?]
	at abw.cL(SourceFile:644) ~[18w11a.jar:?]
	at abv.l(SourceFile:1895) ~[18w11a.jar:?]
	at abw.l(SourceFile:504) ~[18w11a.jar:?]
	at ajs.l(SourceFile:33) ~[18w11a.jar:?]
	at aki.l(SourceFile:257) ~[18w11a.jar:?]
	at abv.S_(SourceFile:1750) ~[18w11a.jar:?]
	at abw.S_(SourceFile:284) ~[18w11a.jar:?]
	at ajs.S_(SourceFile:38) ~[18w11a.jar:?]
	at aki.S_(SourceFile:232) ~[18w11a.jar:?]
	at ajg.S_(SourceFile:252) ~[18w11a.jar:?]
	at aug.a(SourceFile:1617) ~[18w11a.jar:?]
	at se.a(SourceFile:705) ~[18w11a.jar:?]
	at aug.h(SourceFile:1593) ~[18w11a.jar:?]
	at aug.p_(SourceFile:1475) ~[18w11a.jar:?]
	at se.p_(SourceFile:538) ~[18w11a.jar:?]
	at net.minecraft.server.MinecraftServer.w(SourceFile:702) ~[18w11a.jar:?]
	... 4 more
[12:27:24] [Server thread/ERROR]: This crash report has been saved to: /home/~/.minecraft-beta/crash-reports/crash-2018-03-20_12.27.24-server.txt
[12:27:24] [Server thread/INFO]: Stopping server
[12:27:24] [Server thread/INFO]: Saving players
[12:27:24] [Server thread/INFO]: MEMEME lost connection: Disconnected
[12:27:24] [Server thread/INFO]: MEMEME left the game
[12:27:24] [Server thread/INFO]: Saving worlds
[12:27:24] [Server thread/INFO]: Saving chunks for level 'beta10'/overworld
---- Minecraft Crash Report ----
// Don't be sad. I'll do better next time, I promise!

Time: 2018/03/20 12:27 PM
Description: Ticking entity

java.lang.ClassCastException: afn cannot be cast to afj
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at czd.v(SourceFile:155)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Thread.java:748)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Client thread
Stacktrace:
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)

-- Entity being ticked --
Details:
	Entity Type: minecraft:drowned (ajg)
	Entity ID: 132
	Entity Name: Drowned
	Entity's Exact location: 1497.20, 59.55, 830.86
	Entity's Block location: World: (1497,59,830), Chunk: (at 9,3,14 in 93,51; contains blocks 1488,0,816 to 1503,255,831), Region: (2,1; contains chunks 64,32 to 95,63, blocks 1024,0,512 to 1535,255,1023)
	Entity's Momentum: 0.00, 0.43, 0.00
	Entity's Passengers: []
	Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)

-- Affected level --
Details:
	Level name: beta10
	All players: 1 total; [sg['MEMEME'/128, l='beta10', x=1501.62, y=60.68, z=829.02]]
	Chunk stats: ServerChunkCache: 1250 Drop: 0
	Level seed: -1392484337
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-16,64,66), Chunk: (at 0,4,2 in -1,4; contains blocks -16,0,64 to -1,255,79), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 278497 game time, 36115 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 18692 (now: true), thunder time: 63512 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at czd.v(SourceFile:155)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Thread.java:748)

-- System Details --
Details:
	Minecraft Version: 18w11a
	Operating System: Linux (amd64) version 4.4.0-116-generic
	Java Version: 1.8.0_151, Oracle Corporation
	Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 174802872 bytes (166 MB) / 443535360 bytes (422 MB) up to 1060372480 bytes (1011 MB)
	JVM Flags: 5 total; -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
	Profiler Position: N/A (disabled)
	Player Count: 1 / 8; [sg['MEMEME'/128, l='beta10', x=1501.62, y=60.68, z=829.02]]
	Data Packs: vanilla
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.
#@!@# Game crashed! Crash report saved to: #@!@# /home/~/.minecraft-beta/crash-reports/crash-2018-03-20_12.27.24-server.txt
AL lib: (EE) alc_cleanup: 1 device not closed

Uploading another crash. Hoping this gets fixed.

---- Minecraft Crash Report ----
// I just don't know what went wrong :(

Time: 3/21/18 4:20 PM
Description: Ticking entity

java.lang.ClassCastException: afn cannot be cast to afj
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at rp.w(SourceFile:354)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Thread.java:748)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Server thread
Stacktrace:
	at adb.a(SourceFile:41)
	at acz.a(SourceFile:22)
	at adl.a(SourceFile:94)
	at abw.cL(SourceFile:644)
	at abv.l(SourceFile:1895)
	at abw.l(SourceFile:504)
	at ajs.l(SourceFile:33)
	at aki.l(SourceFile:257)
	at abv.S_(SourceFile:1750)
	at abw.S_(SourceFile:284)
	at ajs.S_(SourceFile:38)
	at aki.S_(SourceFile:232)
	at ajg.S_(SourceFile:252)
	at aug.a(SourceFile:1617)
	at se.a(SourceFile:705)
	at aug.h(SourceFile:1593)

-- Entity being ticked --
Details:
	Entity Type: minecraft:drowned (ajg)
	Entity ID: 306595
	Entity Name: Drowned
	Entity's Exact location: 988.15, 59.52, 37.30
	Entity's Block location: World: (988,59,37), Chunk: (at 12,3,5 in 61,2; contains blocks 976,0,32 to 991,255,47), Region: (1,0; contains chunks 32,0 to 63,31, blocks 512,0,0 to 1023,255,511)
	Entity's Momentum: 0.00, -0.03, 0.00
	Entity's Passengers: []
	Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
	at aug.p_(SourceFile:1475)
	at se.p_(SourceFile:538)

-- Affected level --
Details:
	Level name: aqua1
	All players: 1 total; [sg['Stubbornmule'/288228, l='aqua1', x=990.45, y=11.00, z=129.17]]
	Chunk stats: ServerChunkCache: 713 Drop: 0
	Level seed: -9193889702175967349
	Level generator: ID 00 - default, ver 1. Features enabled: false
	Level generator options: 
	Level spawn location: World: (232,64,68), Chunk: (at 8,4,4 in 14,4; contains blocks 224,0,64 to 239,255,79), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 1080016 game time, 1144429 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 58668 (now: false), thunder time: 76528 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.w(SourceFile:702)
	at rp.w(SourceFile:354)
	at net.minecraft.server.MinecraftServer.v(SourceFile:627)
	at net.minecraft.server.MinecraftServer.run(SourceFile:532)
	at java.lang.Thread.run(Thread.java:748)

-- System Details --
Details:
	Minecraft Version: 18w11a
	Operating System: Linux (amd64) version 4.9.0-6-amd64
	Java Version: 1.8.0_162, Oracle Corporation
	Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 561331312 bytes (535 MB) / 1038024704 bytes (989 MB) up to 7265714176 bytes (6929 MB)
	JVM Flags: 2 total; -Xms1G -Xmx7G
	Profiler Position: N/A (disabled)
	Player Count: 1 / 20; [sg['Stubbornmule'/288228, l='aqua1', x=990.45, y=11.00, z=129.17]]
	Data Packs: vanilla
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)

The game crashes too but i set "-Xmx2G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M", it works fine.

Drowneds seems to take too much memory the default game can handle.

Confirming that editing the level.dat file to peaceful and reloading the map fixes the crash, until it occurs again for anyone trying to recover their save.

Just got this bug again while I was working on a zombie grinder. (Spaced out and tried to use a water elevator, forgetting that would turn the zombies into Drowned, so it's completely my fault.) This time, though, it seems to have corrupted my save. Not too fussed about it - I backed up my world a few hours ago, so not much harm done in this case - but it seems like this bug might be even more serious than I initially thought.

I have had this bug occur today whilst going through a cave. I never even saw a drowned mob nearby, and I was at diamond level with no oceans nearby. Yet my crash report reveals the same thing as everyone else here- a Drowned mob.
My world seemed to be corrupt... But I made a few copies in the saves folder, and kept trying to open them. Eventually I got back into my world, ran away from the effected area, and then happened to crash again minutes after.
I have reloaded a world that is now working, but I am not moving until this is fixed! Or, perhaps I should play on peaceful for now...

The weird and important thing to note is that I have created a drowned mob grinder from a zombie spawner far away from where I crashed in this world... And I spent a good hour or so converting zombies to drowned and killing them without a crash happening!
Perhaps... As I think the crash report is suggesting... This has something to do with them swimming, as my farm did not allow them to 'swim around'.

I have 74 crash reports from my server with this crash. We have tried to figure out how to consistently reproduce the bug without success. It seems completely random but happens more on multiplayer than in single player even with only one player online on the multiplayer server. Trying to trigger it on newly created small single player worlds on good computers might prove almost impossible. If you need world download of our multiplayer server or any more crash reports or info about which steps we've taken to try to figure this out I'd be happy to help.