NOPEname

Found the problem!

You need to add a file called "shadow.png.mcmeta" with the following content:

{
 "texture": {
 "clamp": true
 }
}

Also, your texture should not fill the whole image. You need to leave some space at the edge (take default texture for reference).

To me it looks like you simply need to have a "misc/shadow.png" texture in your resource pack for this to happen.

System information:
-MC: Minecraft 1.16.4 (vanilla)
-OS: Pop!_OS 20.04 LTS (Linux 5.8.0-7642-generic, x86-64)
-CPU: Intel i7-4770
-GPU: NVIDIA 1660 Super (driver 460.56)

To me it almost looks like transparent elements are rendered in opposite order in the GUI.

Edit: Setting the Z component of the translation in the GUI display settings to -5 "fixes" the issue.

Thanks, the allignment was is the problem actually.
I got confused since the exact same function did work as I originally expected in an older version. 

So this works as intended!

Is the following problem related to this?
Teleporting a player to an unloaded chunk and summoning an entity in that chunk in the same tick does not work anymore in 1.14.

Example:
Position 0 0 0 is inside an unloaded chunk.
I have the following mcfunction:

tp @a 0 0 0
summon armor_stand 0 0 0

Running the function will result in the player being teleported, without the armor_stand being summoned.

From client to server

I have difficulties to describe the issue, but I tried again and updated the report.

Can confirm for 1.13-pre6

It doen't crash anymore.

log: "Removed resource pack Adventure x16 from options because it is no longer compatible"

Sorry, I mixed up with another crash.

Seems to be fixed in 1.13-pre 3.

Oh, ok.

I don't know if it's helpful, but since my crash report shows different line numbers I will upload it here as well:

---- Minecraft Crash Report ----
// I let you down. Sorry :(

Time: 25.05.18 13:42
Description: Exception ticking world

java.lang.NullPointerException: Exception ticking world 
at sl.n_(SourceFile:422) 
at sl.i_(SourceFile:220) 
at net.minecraft.server.MinecraftServer.w(SourceFile:693) 
at net.minecraft.server.MinecraftServer.v(SourceFile:626) 
at dii.v(SourceFile:156) 
at net.minecraft.server.MinecraftServer.run(SourceFile:531) 
at java.lang.Thread.run(Thread.java:745)

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

-- Head --
Thread: Server thread
Stacktrace: 
at sl.n_(SourceFile:422) 
at sl.i_(SourceFile:220)

-- Affected level --
Details: Level name: #ProjectZelda 1.13 
All players: 1 total; [sn['NOPEname'/96, l='#ProjectZelda 1.13', x=10168.79, y=61.24, z=9919.71]] 
Chunk stats: ServerChunkCache: 1691 Drop: 0 
Level seed: -7581267854039205500 
Level generator: ID 01 - flat, ver 0. Features enabled: false 
Level generator options: {"flat_world_options":"minecraft:bedrock,30*minecraft:water;1;village"} 
Level spawn location: World: (136,31,136), Chunk: (at 8,1,8 in 8,8; contains blocks 128,0,128 to 143,255,143), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511) 
Level time: 29313699 game time, 5000 day time 
Level dimension: 0 
Level storage version: 0x04ABD - Anvil 
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false) 
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace: 
at net.minecraft.server.MinecraftServer.w(SourceFile:693) 
at net.minecraft.server.MinecraftServer.v(SourceFile:626) 
at dii.v(SourceFile:156) 
at net.minecraft.server.MinecraftServer.run(SourceFile:531) 
at java.lang.Thread.run(Thread.java:745)

-- System Details --
Details: 
Minecraft Version: 18w21b 
Operating System: Windows 8.1 (amd64) version 6.3 
Java Version: 1.8.0_25, Oracle Corporation 
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation 
Memory: 904723360 bytes (862 MB) / 1677721600 bytes (1600 MB) up to 3221225472 bytes (3072 MB) 
JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx3G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M 
Profiler Position: N/A (disabled) 
Player Count: 1 / 8; [sn['NOPEname'/96, l='#ProjectZelda 1.13', x=10168.79, y=61.24, z=9919.71]] 
Data Packs: vanilla 
Type: Integrated Server (map_client.txt) 
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

It's actually two data packs: http://www.mediafire.com/file/pnbc7z9fbfwqgu9/2%20datapacks.zip
They there used un-zipped.

They contain invalid commands and block ids (block tag).

Later I updated another world, also containing invald command  functions, so I guess the crash is caused by the block ids.

I am not sure if it is related to this bug, but I dicovered another problem with filling blocks:
I wrote a command function that fills large areas. When I activate it the (singleplayer) server can't keep up and the FPS drop. After a while this happens:
Keep in mind: I was playing in singleplayer!

To me it seems random whether the server starts or gets stuck. I'm launching it with GUI.
Sometimes it works at first try, sometimes it takes 10-20 restarts.
(Windows 8.1; Java 1.8.0_25 64bit)

Can confirm for 18w03b

The same goes for display:{Name:

A resource pack file name contains an invalid character. That makes sense! Is it possible to make mc 1.13 unload invalid resourcepacks while/before launching?

Can confirm for Minecraft 1.12-pre5