This was a VMWare “virtual appliance” with Ubuntu that I was using for testing. As I mostly use Virtual PC or Virtual Server, I found it annoying having to switch to VMWare player to use that specific machine, and I could not be asked to install a new one. So I converted the .VMDK to .VHD format (the other way around than it is described on this article ).
After that, I had to change GRUB’s configuration to inform it that the SCSI disk (/dev/sda1) was all of a sudden become an IDE one (/dev/hda1), and then I also had to reconfigure X.
After that it runs like a charme!!!