Client loads fine in SP, crashes on server join with this output from server:
https://gist.github.com/AnsuzThurisaz/a051c052dbb46ecdc47715e8805d0a83

Client Crashlog:
https://gist.github.com/AnsuzThurisaz/b58f2a40ee4af8be433f6b430b7b0020

Forge 1.12.2 2796
Astral Sorcery 1.10.8

I have a similar crash. We removed the attunement altar placed in the world and there's still a crash:
crash-2018-12-30_21.40.10-client.txt

That client.txt log is not related, @Bluerobin427 . It's for an NPE with perk mapping.

I would agree with you except I started with the same crashlog as @AnsuzThurisaz until we removed the attunement altar. Maybe you're right and I should start a new issue as well?

This happens to me too. I guess i should check issues before updating mods.

Same error on this end

Fixed with 1.10.9