On Sun, 2018-01-07 at 20:37 +0100, Frédéric wrote:
I can start a Plasma session with startx from a terminal screen so
is really sddm. Can I install another session manager?
Yes, though I use sddm with no problems.
However, this does not solve the issue with screen resolution which
1024x768 instead of 1920x1080.
Have you tried xrandr?