After having had big trouble again with Parallels and my Windows 10 guest, I had to kill Parallels from the macOS desktop (Catalina 10.15.5) Although Paralles had been killed there was stil a Winsow 10 process running. Had to kill everything and finally got my system rebooted. Now, after reboot I wanted to started Parallels again. It just hangs there, says "starting" but nothing happens. The system is slowed down terribly to unusable.