OK, Sven had the key of the problem. Forcing the use of Nouveau for the xserver did the tip. Sorry for the inconvenience: as the server started and was mostly working I assumed wrongly that the xserver figured out by itself which card to use. It was halt right only :)
I'll fill a bug repport against xserver-xorg-core as recommended. Stephane -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
