Okay, so the process IDs in question were MMCs. I closed all of them, and the last process ID was Fujitsu's DeskUpdate (the computer I'm using is a Fujitsu workstation). I closed that as well, and now I don't receive any error message at all, just a the Window Picker after a short while. Very strange!
That's weird. I'm not able to reproduce the issue. Can you reproduce it on a different machine as well?