I've noticed that a few addons have copypasted Farmer's Delight Refabricated's porting lib import in their own build scripts for 1.21.1, which includes an out of date build of porting lib. This leads to the mod crashing because Porting Lib has made breaking changes to their code and that i had also updated the version schema to remove the trailing -beta, which leads FLoader to pick up the addon's old version.

For Players

id suggest making a report to any devs. You can search for an old porting lib being loaded by seeing if a mod has a porting lib version with this:
3.1.0-beta+fdrf.1+1.21
trailing it in the modlist within the crash logs. You may search with Ctrl + F.

For Devs

include(modApi("io.github.fabricators_of_create.Porting-Lib:$module:$port_lib_version"))

Replace the above with: modApi("io.github.fabricators_of_create.Porting-Lib:$module:$port_lib_version")

If you are on 1.21.0

Update to 1.21.1, the current Porting Lib doesn't load on base 1.21. It won't hurt, 21.1 is only a hotfix version.

Porting Lib NoClassFoundException

Update to Fabric Loader 0.16.7 or above.