Minecraft version: 1.20.1
Java version: 17.0.11
NeoForge version: 47.1.106

Log link: https://mclo.gs/wGrwsFo

I'm not completely sure, but it seems like Thermal's Blizz mob is conflicting with open-parties-and-claims. This issue seems to happen when a Blizz enters a claim and uses its attack (it places a snow layer on the ground), but since it's in a claim, it's not allowed, and it crashes my game. At least, that's what I gathered from the log. I was using FTB Chunks, but I wanted to modernize a bit, so I decided to contribute to this mod. Of course, I didn't experience such a crash when using it..

I experienced it twice, and my solution is to deactivate the mod, kill the mob, and then reactivate it.

It's just NeoForge 1.20.1 not actually supporting Forge mods properly unfortunately. Same as #571

I suggest you switch to Forge 1.20.1, if possible.