If you updated from 1.20.4 to 1.21.1, your old config files might be causing a "Null Pointer Exception." Go to your .minecraft/meteor-client/ folder. Backup your accounts.nbt (to save your logins).
Minecraft 1.21.1 requires Java 21. If your launcher is forced to an older version, the addon will fail to load. meteor rejects addon 1211 work
The transition to Minecraft 1.21.1 introduced several "under the hood" changes to the Fabric Loader and the way Minecraft handles internal networking and rendering. Because Rejects is an addon to Meteor Client, it requires two layers of compatibility: it must work with the base Meteor Client version AND the specific version of Minecraft you are running. Common reasons for it not working include: If you updated from 1