Today I installed a fresh bullseye system with fvwm and also run into this issue. Several actions on the desktop resulted in this error message and fvwm terminating. Restarting fvwm via + "Restart fvwm" Restart fvwm in the menu reproducibly triggered the issue.
I installed fvwm 2.6.8-1 from buster, but the problem stayed there, so this may be some regression triggered by some other component of bullseye in combination with different versions of fvwm. Since loosing the desktop with all open windows drives me crazy (and I don't want to learn the behavior of some other window manager), I installed fvwm3 and used this one and except that I have to adapt my configuration to use Colorsets (instead of Colors) now, everything seems to work as expected and I can no longer reproduce the issue with fvwm3. Greetings Roland
