With only mod menu 11.0.0-beta.1 installed the game keeps crashing during world generation.
Here's a copy of the crash log
crash-2024-06-14_08.53.01-server.txt

Had the same issue. This occurs only when Fabric API is not standalone installed (only occurs with the jar-in-jar modmenu's fabric api).

As a temporary workaround, installing Fabric API version 0.100.1+1.21 solves this. Logs: https://api.mclo.gs/1/raw/H4OKg5o

It's primarily because ModMenu was compiled with a previous version of Fabric API, so the jar-in-jar version is outdated...

Mod Menu for 1.21 is now out