Thanks, I will try this, but I doubt this will help. Raspberry PI OS and Emulation Station recognise the display correctly as they feed it with 1280x800 automaticaly. RetroArch seems to miss this information or ignores it.
Edit: With hdmi_ignore_edid=0xa5000080, resolution in raspi-config = 1280x800 and no resolution set in RetroArch everything works fine. I still do not realy get why, but thanks a lot @mitu!