Very recently installed Linux Mint 21, Mate Edition.
Installed Gimp 2.10.30.
Opened Gimp, unchecked Windows:Single-Window Mode
and was immediately logged out of my current user session.
Had to log back in.
I have been using Gimp on Ubuntu Mate 20.04 for a couple years
with no problems.
Has anyone heard of this problem or could offer help?