Modpack: E6E
Latest log: https://pastebin.ubuntu.com/p/KYVXq5rYfB/ (spam...)
Crash report: https://pastebin.ubuntu.com/p/7YfBhk95FN/

Sometimes maybe. I created several worlds but only this first one got crashed.

Seems most likely to be some mod accessing NBT from multiple threads. Guess I will have to make CanonizingStringMap thread-safe. Does it happen consistently or only sometimes?

I think it's most likely some mod interaction that mutates this from multiple threads and making the NBT map thread-safe would reduce performance in all other scenarios. So I think I'll leave this one unfixed, doesn't seem to have been reported commonly.

After stumbling across a similar issue in 1.20 with a completely different mod I tracked this down to Forge most likely firing a config event to the same mod concurrently. Should be fixed in 5.0.4 and newer by 41b71c5.