Once, in a Debian work machine. Did as it said, worked flawlessly afterwards.

I was able to fix it on my machine by switching from the fedora to the breeze theme

Yes, I get this regularly in my piece of shit work HP laptop running Ubuntu. It doesn’t handle my USB dock well.

Try to do as it says, although it sometimes gets the session number wrong and maybe the F-key too.

You probably ran an update before this and updated the screen locker. Then the OS was in a mismatched that caused the screen locker to break.