Since the upgrade to 20.04 whenever my laptop screen is locked, the issue is that the screen keeps on showing the last content, and the mouse can be moved, but it cannot be interacted with. The screen is effectively locked but there is no way to unlock the screen. If I close the laptop lid and re-open it, the mouse cursor remains visible but the rest of the screen runs black.

Initially I thought it was an issue with suspend/resume and my graphics setup and/or the fact that I'm using a dual monitor setup and/or in combination with running android emulators through Android Studio.

Then one time when locking the display with Ctrl-Alt-L I noticed that "nothing happened" - the locked screen screen did not come up. So I tried again and still nothing happened.

I then tried to interact with the screen and realised it was locked, just still visible. So switching to a terminal Ctrl-Alt-F2 allowed me to look at processes.

Since at this point I normally am forced to do a power cycle I am not to concerend with killing processes. I tried killing processes one by one. On killing "kscreenlocked_greet" the screen revived and I was met with the normal unlock screen.

I am now able to reproduce the issue consistently using the following steps

Log in
Lock screen with Ctrl-Alt-L
-> note screen is locked and cannot be interacted with but the display remains visible, eg showing the windows that was visible at the time Ctrl-Alt-L was pressed.
Switch to a termina Ctrl-Alt-F2
Log in
killall kscreenlocker_greeter
Switch back to GUI Ctrl-Alt-F1
Locked screen is now visible and the login password box appears when I move the mouse. After unlocking everything appears normal until I lock the screen again.

I hope this workaround / steps will be useful to other people!