SUMMARY
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My laptop works fine without any external monitor, but for some months now, I have suddenly problems with my external monitor, which is connected over USB-C. If my monitor is plugged in during boot, is just shows a black screen and I see a movable cursor. The only workaround is to plug my external monitor off, reboot, log in and then I can plug in my external monitor again. The desktop switches then 3x from my external monitor to my laptop and vice versa until it is stable for some time, randomly this happens again but at least only once, it also hear a sound over my speaker. I already tried XWayland and X11, it is still the same behaviour. I also wiped out all my KDE configurations.

STEPS TO REPRODUCE
1. Plug in my external monitor during or before boot
2.
3. 

OBSERVED RESULT
Desktop is not usable. Black screen with no content except a movable cursor. Plugging off the external monitor does not help, as my laptop monitor is still turned off.

EXPECTED RESULT
If the external monitor is plugged in, KDE shall use it.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Kernel 6.1.18
(available in About System)
KDE Plasma Version: 5.27.3
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION
The whole setup worked for years some months before without any problems.