A lot of players on our server have been crashing recently. This has been happening for weeks. We did not immediately understand what is causing it, but it does appear the anti xray mod is the culprit: when it removes, no more crashes occur.

I have attached fifteen crash logs, collected over a span of three days. The issue is extremely reproducible on one particular world. Clients crash with a variety of mods, although the given stack tracer always include sodium and lithium. Other than that, there is a variety of mods present. Players have reported it crashes on Vanilla too, but I have not tested this myself.

All errors look similar to this:

net.minecraft.class_6558: Missing Palette entry for index 51.
	at knot//net.caffeinemc.mods.lithium.common.world.chunk.LithiumHashPalette.missingPaletteEntryCrash(LithiumHashPalette.java:140)
	at knot//net.caffeinemc.mods.lithium.common.world.chunk.LithiumHashPalette.method_12288(LithiumHashPalette.java:134)
	at knot//net.minecraft.class_2841.sodium$unpack(class_2841.java:2059)
	at knot//net.caffeinemc.mods.sodium.client.world.LevelSlice.unpackBlockData(LevelSlice.java:228)
	at knot//net.caffeinemc.mods.sodium.client.world.LevelSlice.copySectionData(LevelSlice.java:193)
	at knot//net.caffeinemc.mods.sodium.client.world.LevelSlice.copyData(LevelSlice.java:179)
	at knot//net.caffeinemc.mods.sodium.client.render.chunk.compile.pipeline.BlockRenderCache.init(BlockRenderCache.java:50)
	at knot//net.caffeinemc.mods.sodium.client.render.chunk.compile.tasks.ChunkBuilderMeshingTask.execute(ChunkBuilderMeshingTask.java:70)
	at knot//net.caffeinemc.mods.sodium.client.render.chunk.compile.tasks.ChunkBuilderMeshingTask.execute(ChunkBuilderMeshingTask.java:52)
	at knot//net.caffeinemc.mods.sodium.client.render.chunk.compile.executor.ChunkJobTyped.execute(ChunkJobTyped.java:45)
	at knot//net.caffeinemc.mods.sodium.client.render.chunk.compile.executor.ChunkBuilder$WorkerRunnable.run(ChunkBuilder.java:220)
	at java.base/java.lang.Thread.run(Unknown Source)

Similarly, on the server, various mod configurations have been used, although the common denominator is:

• Async
• Alternate Current
• Carpet
• Ferrite Core
• Krypton
• Lithium
• Polymer
• Spark

I have removed the bottom halves of the crash logs to preserve anonymity of the players.

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Can you please try to reproduce this without async.

That seems to have been the culprit, yes. Still, we will likely revert to re-enabling it, as it gives a massive performance speed-up.

Async is doing very cursed stuff and it doesn't feel like they know what they are doing.
For example the first thing I saw: AllayEntityMixin, this is not how you use locks.
You can try to create an issue on their repository, but I won't provide much support with this mod installed!