I applied the #37 fix to start mixer localhost in 2 instances of Blender 3.0.
Connected in both instances.
Created New Room in main instance.
Clicked Join Room in second instance and second instance crashed.
Attached is crash log if it helps. I replaced my name with USER in the filepaths, otherwise nothing is changed.
blender.crash.txt
Windows 10
Mixer v1.0.1