Thanks for the reply!
I tried to draw a straight line on the right side, and the result was the “squared” line. On the left side, one can see that the major part does not respond. Having followed exactly your instructions, Stefan, I can now, after having selected “reboot” as the last step, neither turn my phone on anymore nor get back into the factory mode. The only thing the phone does when I press the power button is show a black screen for half a second.
So hardware problem and contact the support?