KDE forgets about display resolution in 8.1-release

[Friedemann Becker]

Hello,

every time I boot, there's a display resolution of 1600x1200. I can change
it via Start/KDE => Settings => System Settings => Display => Size &
Orientation, after login out and in again, it's still the same, but after
rebooting it's lost.
I don't want to put a fixed resolution in /etc/X11/xorg.conf, so is there a
way to make KDE remember my settings? where do I have to start searching?

Greetings,
Friedemann