crash-2020-05-14_20.45.37-client.txt

Could this be caused by an old Fabric (API?) version?

In general, yes, could be caused by old fabric loader, old fabric api, old optifabric or old optifine, don't see how it would be caused by RM though.
Most likely a mismatch between fabric api and optifabric.

Closing cause outdated RM and other mods.