Getting a weird crash on launch
https://pastebin.com/p1SVrEkX

That is real weird, reading the code behind that, a final field is null... so something extra sketchy is going on.

Would posting the debug log help?

Is there any way to find out what's causing this, besides removing mods one by one?

No, because whatever is messing with it has no accountability in that log (so it's probably a mixin or smth).
I've never seen this in either Valhelsia or ATM5, so you could cross-check with those modlists and try removing things that are unique to this pack.

Thanks to MoreCrashInfo, I found out it was from Silent's Gems