Crashreport
Forge 1.20.1 - mclo.gs
debug
Forge 1.20.1 - mclo.gs
latest.log
Forge 1.20.1 - mclo.gs

I found "Cannot read field "f_91074_" because "atomicstryker.infernalmobs.client.InfernalMobsClient.mc" is null" in latest.log and I think that should link to this mod.

Ok. Thank you very much! :>

[net.minecraft.util.thread.BlockableEventLoop/FATAL]: Error executing task on Client

java.lang.NullPointerException: Cannot read field "f_91074_" because "atomicstryker.infernalmobs.client.InfernalMobsClient.mc" is null
at atomicstryker.infernalmobs.client.InfernalMobsClient.onEntityJoinedWorld(InfernalMobsClient.java:36)

Aye, this is the the relevant message.
So somehow you have EntityJoinLevelEvent before ClientPlayerNetworkEvent.LoggingIn triggered
Using some SpongeForge amalgamation...

Well, let me just add a late init if this happens for you

e810b3b

5b23554

new versions for 1.20.1 and 1.21 are pushed, thanks for the report.