I'm not entirely sure what's causing the problem, but this is what happens when using the mod for me: https://paste.ee/p/PNkvN. Mods that I suspect might cause the crash are Rubidium (unlikely but possible), Starlight, Elytra Bounce (again unlikely but possible), Better Third Person and/or Elytra Physics.

This seems to be an incompatibility with Debugify, caused by how forge handles bundled modules. I'll see what I can do about it.

Fix will be pushed in the next update.

Alright I did a dumb with 2.0.1 and the "fix" actually broke the entire mod. Reopening this as I rolled back those changes.

There is one thing that is very easy to overlook: the package name in the SPI file (in this case is META-INF/services/javax.annotation.processing.Processor) will be also added to the module.
This mod only relocates the MixinExtras itself, but does not relocate the SPI.

Actual fix being published in 2.0.3, this time it shouldnt break anything.