Astral Sorcery 1.12.9
Forge 31.2.41

Crash Report: https://gist.github.com/Vaelzan/51cf9fc83f593fdf132897b93267a206

Report seems pretty clear - seems the tier of a crystal was negative somehow and there are no checks there for that.

I have no idea how the tier ended up negative, as I wasn't around or involved in this, I'm just the server admin / pack creator, but a check to make sure the tier is positive would fix this crash even if the other issue persists somewhere.