@ Kronos
I know how the power button works when the system is intact. In my case the screen turned black, no reaction to mouse movement, key press, caps lock etc., and when pressing the power button sometimes(!) the Ambient screen reappeared. That's not how the power button should work. Maybe it was more the physical pressure in that area of the housing that caused the display to come up again.
At the next boot it lasted only two seconds in Ambient until the screeen switched to black. When I now boot it anew (again and again), the display and front LED both stay turned off. Possibly some flux could help. I don't have the tools at hand for now.
Because the Caps Lock key also shows symptoms, I suspect the PCB to have some defect.

I won't have the time to open it at least before next week. I won't have spare parts available until then. I will be in Mönchengladbach and Neumünster but hope to find a solution before the Neumünster weekend. Currently I use a borrowed PB.


@geit
I pushed - there were no changes visible. Also the turned off Caps Lock and front LED (that permanently stays off now) speaks against a (sole) GPU issue.