Re: The screen locker is broken and unlocking is not possible anymore
I have had that happen. Using "loginctl unlock-sessions" did work for me (I had to run that as root).
Another option would be: CTRL-ALT-BACKSPACE (twice). That should crash the X-session and allow you to login to it again.
I normally turn off the screen locker (computer is in a sufficiently secure location). And then this never happens.
openSUSE Leap 15.4 Beta; KDE Plasma 5.24.4;
testing Tumbleweed.