This bug happened starting from Plasma 5.12.3 (or maybe .2). I remember perfectly that it worked on the initial version of Plasma 5.12. What happens: With an external monitor the screen will suddently become black and the system will go to the login screen (which is correctly visualized). Now I use the live version of KDE Neon. When I start the system without the external monitor HDMI attached, it correctly loads. As soon as I attach the HDMI the screen becomes black and then loops between the initial white KDE logo on a black background and a total black screen. I use the live distro KDE Neon as downloaded from the website, without any modification. My GPU is: AMD HD 7950, CPU: Intel i5 3570k. The problem is 100% reproducible so I can provide logs if you tell me the command to input in the terminal.
Is this on X11 or Wayland?
It is on X11
Reassigned to ksmserver which holds the X11 session.
I'm closing this issue as I switched back to plasma and no more shows up in newer versions.