Forge 1.12.2 build 14.23.5.2854
Fossils and Archeology Revival version 8.0.4
Custom mod pack

Player said he was in twilight forest dimension when crash happened.
Not sure if he was killed by tar slimes, he said he wasnt killed, but either way he had an interaction with tar slimes and it crashed the server and it crashes every time he joins.

Crash report:
https://pastebin.com/ZPNARgck