It means the process respinsible for unlocking the screen has crashed. I’ve seen it happen to me a lot too under some circumstances, but my error message is in English.

It’s telling you to switch to a console and manually unlock the session.

The session is locked for unspecified reasons and can thus not be shared. You are supposed to execute the command in the middle paragraph in a new terminal session.

It’s a bug in KDE Plasma. Iirc it will be fixed in KDE 6

It’s basically saying that the lock screen could not be started. It often happens when certain parts of the system, like SDDM, are updated without rebooting. From what I’ve heard, it should be fixed in Plasma 6.

It’ll likely go away with an update. But you can always check xsession log, dmesg, etc to see if there’s a hint on why the screen locking process is crashing.