Might be incompatibility with some other mod. 1.20.1 (NeoForge 47.1.79), apoth 7.0.5.

Crash log: https://pastebin.com/wLfDxBg2

EDIT: replaced with pastebin link.

Removing vanillafix-1.0.0-forge-beta.jar fixed the problem. Seems like Apotheosis is not compatible with https://www.curseforge.com/minecraft/mc-mods/vanillaicecreamfix by @repletsin5
(https://github.com/repletsin5/VanillaIcecreamFix).

Yeah they're shipping a cursed @Overwrite of the entire anvil update method.
https://github.com/repletsin5/VanillaIcecreamFix/blob/forge/forge/src/main/java/me/replet/client/other/AnvilScreenHandlerMixin.java#L42

Closing as mod conflict.