One more datapoint : disabling KMS by booting with "nomodeset nouveau.modeset=0" allows me to open successfully an X session, without gettng the problem described above.
However, it turns out that the session is run bu the vesafb driver (see enclosed Xorg log of my current session), which is crappy due to X being unable to pick up my monitor's specs from the hardware. HTH, -- Emmanuel Charpentier
Xorg.0.log.gz
Description: application/gzip
