OK, thanks to Cédric Dupont on an Ubuntu users' mailing list, here is the solution which worked:
- Boot on a install CD
- mount /dev/sda1 /mnt
- chroot /mnt
- aptitude update
- aptitude dist-upgrade
Apparently, the upgrade to the new system was not completed (Cédric noted that the kernel in the report was 3.5 while Ubuntu Raring Ringtail uses 3.8). aptitude was enough to complete it properly.
A few details: in order to use /dev/sda1 as my new root file system, I also had to mount /dev/pts and /run and /run/lock, and to create special node for the root partitipn ('cd /dev; mknod sda1 b 8 1')