If you continue to have this issue … I had a very similar issue which started randomly a few months back - ultimately fixed it by … disabling the image on the dom0 screensaver as well as dom0 background, just made them plain color grey. Worth a try!
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Thanks! Definitely worth a shot 
Something similar for me. I use KDE, which is why I’ve said nothing; the most likely result would have been a “go talk to KDE”. Sometimes when I come back to a locked screen, moving the mouse, trackpad, etc. does not bring the screen back (it’s just black). The mouse pointer moves, but the screen is black.
I’ve found that plugging in (or pulling out) a thumbdrive will bring the lockscreen back.
I’ve also noticed, at random times sys-usb (or is it dom0? Will have to pay more attention in the future) seems to try to do something with a thumb drive left plugged in but unattached to any vm.