After an automatic logout I came back to my PC and entered in my password after which I get a screen a bit like the login screen with an unlock button under the user pic. The unlock button does nothing and I am forced to shutdown and reboot. I lost hours worth of work as a result.
Searching for this I found references to a bug that was supposedly fixed around KDE 5.25.
Im on KDE 5.27
Ive looked at SDDM settings - behaviour I cannot see anything relevant to the issue there. Automatically log in is unticked.
Hum, could be, although since Plasma allows to tick both options I'm sure they would not allow it to break like this (unless there's another bug around).
But yes disabling all Lock screen feature means no Automated Lock after x minutes and no Lock after wake up. This is exactly how I am setup on my machine. And I still got the option to MANUALLY lock the screen if I want to.
So even when manually locking the screen you get the issue when trying to unlock it?
My next step in such a situation would be to continue the process of elimination.
Let's see if it's related to the SDDM Theme.
The best test is to switch your SDDM Theme to default Plasma Breeze, reboot to make sure, login, then lock the screen and try to unlock it.
If it works fine as expected, my next validation would be to try another 3rd party Theme (other than Sweet) and see if the issue reproduces.
If the problem is with 3rd party Themes in general that's one thing.
If the problem does not reproduce with at least one other Theme, that's something else.
Very easy to test this and the results will be informative as where to troubleshoot deeper.