I'm playing with only a few mods on a server, I got mousetweaks but it sometimes (maybe 75% of the time) throws this error: "Internal Exception: java.lang.NoClassDefFoundError: com/google/common/collect/CollectSpliterators$1"

Restarting the game can fix it or not fix it, and removing the mod always fixes it

It works fine for the other person on the server.

Using 1.21.4-forge-54.0.17

Can you show the full backtrace? This error doesn't seem related to Mouse Tweaks.