Quilt version 0.26.3 for 1.20.1
Most recent version of quilted fabric api used.
It happens whether or not I use other mods. Legacy4j requires fabric api.
I've checked the issues page for legacy4j and they claim that it's quilted fabric api being incompatible. Heres the issue page in legacy4j: Wilyicaro/Legacy-Minecraft#169
Heres a pastebin of the error report
https://pastebin.com/y0vQ140S

They use fabric internals: net.fabricmc.fabric.impl.resource.loader.ResourceManagerHelperImpl.registerBuiltinResourcePack.

From looking at the code, it seems they store a resource pack in their mod in a non-standard location, and are calling the implementation method to get around this. The method they should call is this and move their resource pack to the standardized location.