@lakotaubp Stable
@AppLee I know, but if nothing other works ...

@all As nothing worked I reinstalled UBPorts. Now the file mananger seems to be fine.

@all
Think the problem seems to be solved - in one or the other way ...
As I wrote the red buttons are not functional - but only if the OnePlus is in portrait format. When it is in landscape the red buttons are functional ...
If I draw lines in a graphical app there are blank parts on the screen, too.
It seems I've got a problem with the hardware.