Some entities do not render inside of spawners and producing error log spam in console, potentially causing lag

Confirmed, also the log is spammed with this:

[01:00:15] [Render thread/WARN]: Exception loading entity: 
s: Loading entity NBT
	at aol.f(SourceFile:1642) ~[1.16-pre5.jar:?]
	at aop.a(SourceFile:467) ~[1.16-pre5.jar:?]
	at aop$$Lambda$4201/93583133.accept(Unknown Source) ~[?:?]
	at v.a(SourceFile:401) ~[1.16-pre5.jar:?]
	at aop.a(SourceFile:466) ~[1.16-pre5.jar:?]
	at aop.b(SourceFile:529) [1.16-pre5.jar:?]
	at aop.a(SourceFile:510) [1.16-pre5.jar:?]
	at bpc.d(SourceFile:256) [1.16-pre5.jar:?]
	at eei.a(SourceFile:22) [1.16-pre5.jar:?]
	at eei.a(SourceFile:12) [1.16-pre5.jar:?]
	at edw.a(SourceFile:107) [1.16-pre5.jar:?]
	at edw.b(SourceFile:96) [1.16-pre5.jar:?]
	at edw$$Lambda$4990/1816104835.run(Unknown Source) [1.16-pre5.jar:?]
	at edw.a(SourceFile:128) [1.16-pre5.jar:?]
	at edw.a(SourceFile:96) [1.16-pre5.jar:?]
	at ebx.a(SourceFile:1236) [1.16-pre5.jar:?]
	at ebs.a(SourceFile:717) [1.16-pre5.jar:?]
	at ebs.a(SourceFile:540) [1.16-pre5.jar:?]
	at dlx.e(SourceFile:1021) [1.16-pre5.jar:?]
	at dlx.d(SourceFile:654) [1.16-pre5.jar:?]
	at net.minecraft.client.main.Main.main(SourceFile:215) [1.16-pre5.jar:?]
Caused by: java.lang.ClassCastException: dym cannot be cast to zc
	at ayk.a(SourceFile:260) ~[1.16-pre5.jar:?]
	at aol.f(SourceFile:1633) ~[1.16-pre5.jar:?]
	... 20 more

Well, this bug appears for all mobs that have their readAdditionalSaveData(CompoundTag) method overriden + a call to readPersistentAngerSaveData which casts the Level to a ServerLevel. As renderers are client sided executing this method's code causes a ClassCastException (ClientLevel cannot be cast to ServerLevel).

A simple server side check should fix the bug.

can confirm for 20w30a

Evoker Fangs & Experience Orbs are also affected

@Algeseven – could you possibly clarify in which class the server-side check needs to be implemented? I've been patiently waiting for a fix for this but it seems like we're not getting one until 1.17 which is unacceptable in my opinion. I'm trying to create a patch to fix this myself as some features on my server depend on some of these spawners, and I'm having trouble determining where this check should go. Thanks in advance.

@Mike - A modder named Draylar made a mod on fabric to patch this bug. Using mixins, he added a check to make sure the world is not a client world before angerFromTag method gets called (yarn mapping names)

You can either use his mod or check his code to create a fix for your server. As for Mojang devs reading this, this is all that is needed to fix the bug and so far, I have not seen any side-effects of this patch as I use it with my own mods as well.
https://github.com/Draylar/angerable-patch

I have fixed this but the fix does not include Evoker Fangs or Experience Orbs, which have a different cause and do not cause log spam. Please raise a separate issue for them.

I have created a report for Evoker Fangs. See MC-199429 Experiance Orbs are not affected but just really small. See: MC-196141

Issue still happens in 1.16.4 pre 1

This should be fixed for 1.16.4, not 1.17. It is critical.

This bug still affects release 1.16.4 (vanilla client and server) using the following custom spawner:

/setblock ~ ~-1 ~ minecraft:spawner\{SpawnData:{id:iron_golem},SpawnCount:3,SpawnRange:4,RequiredPlayerRange:25,Delay:20,MinSpawnDelay:9600,MaxSpawnDelay:9600,MaxNearbyEntities:25} replace

Spamming the client logs with thousands of warnings about:

Exception loading entity: u: Loading entity NBT at aqa.f(SourceFile:1663) at aqe.a(SourceFile:473) at x.a(SourceFile:402) at aqe.a(SourceFile:472) at aqe.b(SourceFile:535) at aqe.a(SourceFile:516) at bqz.d(SourceFile:254) at ecp.a(SourceFile:22) at ecp.a(SourceFile:12) at ecd.a(SourceFile:107) at ecd.b(SourceFile:96) at ecd.a(SourceFile:128) at ecd.a(SourceFile:96) at eae.a(SourceFile:1260) at dzz.a(SourceFile:727) at dzz.a(SourceFile:546) at djz.e(SourceFile:1048) at djz.e(SourceFile:681) at net.minecraft.client.main.Main.main(SourceFile:215) Caused by: java.lang.ClassCastException: class dwt cannot be cast to class aag (dwt and aag are in unnamed module of loader 'app') at bai.a(SourceFile:170) at aqa.f(SourceFile:1654) ... 18 more

As a consequence, user logs can grow very large.

Still affects 1.16.5

Added, but note that 20w45a is a 1.17 snapshot, and fixes from those aren't present in 1.16.5; that you can reproduce in 1.16.5 doesn't mean that the bug isn't fixed.