After restoring xorg.conf to default (removing virtual resolution), X-Server / GDM works again.
Now changing resolution of the external display works without problems, even with System -> Preferences -> Appearance -> Visual Effects: normal.
But the other problem remains, that xrandr does not work (nothing happens).
I attach Xorg.0.log and Xorg.0.log.old agagin.
After restoring xorg.conf to default (removing virtual resolution), X-Server / GDM works again.
Now changing resolution of the external display works without problems, even with
System -> Preferences -> Appearance -> Visual Effects: normal.
But the other problem remains, that xrandr does not work (nothing happens).
I attach Xorg.0.log and Xorg.0.log.old agagin.