Minecraft 1.16.5
Forge 1.16.5-36.2.23
infernalmobs-1.16.2.1
TheAbyss2+2.2.3-1+1.16.5
https://www.curseforge.com/minecraft/mc-mods/the-abyss-chapter-ii
Server Crash Log: https://pastebin.com/gP4zpWVq

Only when installed on a server, infernal mobs is messing with the IceSkeleton entity of The Abyss 2 and crashing the server. It happens when a player enters the frost dimension of The Abyss 2 and IceSkeletons appear. It´s working fine in singleplayer. When uninstalling infernal mobs it is no longer crashing.

Yes. The Abyss 2 works without a problem in multiplayer when infernal mobs is not installed. This error/crash does not appear then.

Just from the looks of it, this appears to be a problem with this abyss mod. Does that mod work in multiplayer without Infernal Mobs? The crash claims its entity class is not dedicated server compatible.

Have you reached out to the dev of that mod? I feel they have clientside code in a server class, and i don't see how Infernal Mobs could fix this

EDIT: Just for understanding, what basically happens is that IceSkeleton is spawned into the world, Infernal Mobs asks "hey whats your name" and the abyss mod goes REEEEE I CRASH NOW

^^ Yes, I have talked to the dev before here:
Y3Z0N/theabyss#36