Problem fixed. I ran the foolowing command to determine obsolete packages. After removing them, the upgrade succeeded.
$ cat /var/log/dist-upgrade/main.log | grep Obsolete -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1385281 Title: Cannot upgrade ubuntu 12.04 precise to 14.04 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/1385281/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
