Hi Andreas,
Please try Webconverger 12.3 in debug mode.
Type `xrandr -q` to figure out the connected output name.
In my case it is HDMI1.
Then try:
xrandr --output HDMI1 --rotate right
If it that works you need to then put in your config:
xrandr=--output%20HDMI1%20--rotate%20right
-o is the old way of doing it I'm told:
http://lists.x.org/archives/xorg/2012-April/054475.html
Kind regards,