Placing lots and lots of falling sand really quickly causes a crash.
Removing WI Zoom fixes the problem.
Possibly a rendering issue (21w10a uses new rendering), but who knows?
https://paste.ee/p/STWC2 is the log. (Pardon all the Forge mods listed, they are disabled and not loaded by Fabric anyway.)

This is a known bug in Fabric API version 0.32.1. It has been fixed yesterday. Please update to Fabric API v0.32.2.