richter_f@gmx.at writes:

> I thought that the ddc module could be an error source but the option
> "NoDDC" 
> brings no other (better) results. I also tried to put in values for
> DisplaySize - the 
> same result. 

I don't know why it's auto-detecting improperly, and I'm not sure that
DDC is how laptops implement screen detection, but you can directly
configure the panel size, which is what I do with the Rage 128
Mobility LF on my iBook. My XF86Config contains:

Section "Device"
    Identifier  "Rage 128"
    VendorName  "ATI"
    Driver      "r128"
    BusID       "PCI:0:16:0"
    Option      "PanelWidth" "1024"
    Option      "PanelHeight" "768"
EndSection

Tweak to fit your setup; you probably don't need the BusID part.

        - Nathan