Error Root:
Caused by: java.lang.ClassCastException: class net.minecraft.class_1091 cannot be cast to class malte0811.ferritecore.mixin.mrl.ResourceLocationAccess (net.minecraft.class_1091 and malte0811.ferritecore.mixin.mrl.ResourceLocationAccess are in unnamed module of loader net.fabricmc.loader.impl.launch.knot.KnotClassLoader @21a947fe)
at net.minecraft.client.util.ModelIdentifier.handler$cog000$constructTail(ModelIdentifier:1524)
at net.minecraft.client.util.ModelIdentifier.(ModelIdentifier:30)
at net.minecraft.client.render.item.ItemRenderer.(ItemRenderer:65)
... 20 more

Full Log:
https://pastebin.com/CFGkJFnp

I changed some mods (I don't remember which ones), and now it no longer crashes.

Is there perhaps an unknown incompatibility with a mod?

This is a typical issue when some mod loads the Identifier/ResourceLocation class too early (at a point where no MC classes should be loaded). The only way to debug it (besides a binary search on mods) is for me to load the pack under a debugger, but I'd need the full pack for that. Is it available anywhere?

The pack isn’t available at the moment as it was for personal use and I made changes that ended up fixing it. Thanks for the response!