Despite being marked as compatible for Quilt on Modrinth, as of QSL+QFAPI 9.0.0-alpha.5+0.96.11-1.20.4, the mod is incompatible with Quilt.

Crash Report: https://pastebin.com/qrEqymvx
Log: https://mclo.gs/limj9O5

If it would be possible to make a fix for Quilt, i feel it'd be appreciated by users. Or please go and unmark the versions as Quilt Compatible

Legacy4J is compatible with Quilt, but not with the Quilted Fabric API, which for some reason changed support for builtin Resource Packs, you must use the Fabric API

Ideally you should report this to the Quilted Fabric API issue tracker as it IS an incompatibility with Fabric API

QuiltMC/quilted-fabric-api#168 (comment)