I've found a problem that seems to correlate when the renderer class is called. The class in charge of Bibliocraft's armour stands seems to conflict somehow with the Regeneration skin changing handler. Bibliocraft for 1.12 hasn't been updated since last year, but I can hope that there is still some way to manage a fix.

Here's the crash report: https://paste.dimdev.org/azunetavil.mccrash

From the looks of things, it seems like it'll be a easy fix :)
Keep your eyes our for 1.8.0, drops this coming week

I actually still have that crash after 1.8.0 :/ I've tried disabling new mods, but I still had the crash afterwards. I'm going to just disable Bibliocraft and loose what little decor and shelves I had from that mod. It's not worth it if it keeps crashing on me, especially when I have alternatives.

In case the current crash report is different or telling, here's the paste link: https://paste.dimdev.org/gowakixeze.mccrash

Should be fixed in: 877d6e2

https://www.curseforge.com/minecraft/mc-mods/regeneration/files/2766859