I have a Win7 VM in Boxes. After I upgraded to Fedora 34 everything worked like a charm, except that my VM inside Boxes refused to start. After tinkering a little bit, I tried cloning my VM. The newly created VM started successfully. I was able to use the VM for a while, then I suspended the VM and quit boxes. From than on, I cannot start Boxes at all. The Boxes window appears for a moment and then it disappears.
Any help appreciated.
Edit: I deleted the old VM immediately after seeing that the new VM was working.