I'm fully unsure what is causing this. I used to be able to circumvent the issue by running an older version of forge (36.2.2), but other mod updates forced me to update to a newer version of forge, which causes some kind of mod incompatibility. All I could dig out from the log is that's in relation to injection into method_17410 for the fullExperienceCost method.

I am running on the latest published build (2.2.1-exp1) but this is happening on older versions as well.
Forge version 36.2.19

debug.log - https://gist.github.com/GAsplund/cb9d23b9c6379f054b284f4ad8b5da04

Duplicate of #357

Something about your environment is promoting this soft failure into a hard crash.

Set full_experience_cost in features.ini to banned as a temporary workaround.