https://gist.github.com/Zsashas/37ba3604feab4964afe3eee6d2b8d14a

Betweenlands is probably not registering one of their entities, that's the only reason I can think could cause the crash. You can work around the issue by adding:

HEE:System:DisableIntegration { "modid": "arsmagica2" }

to the IMC section of the config file.

What integration is there with AM2, anyway? What would I be missing out on?

The integration works around an AM2 crash, but I think it was fixed in AM2 so it hopefully wouldn't crash (but AM2 aspects might not work on some of the HEE mobs).

Yes, that could be the case. We accidentally removed an entity from the registry and that was fixed just recently (Angry-Pixel/The-Betweenlands@9e8a6c5). I'll test it today if I have time.

Just realized that it's this that's causing the error: https://github.com/Angry-Pixel/The-Betweenlands/blob/master/java/thebetweenlands/entities/properties/list/recruitment/EntityPropertiesRecruit.java#L42
I'll fix it later today.

Fixed on Betweenlands' side.