Comment 4 for bug 63205

Thanks for your bug report. Your problem is most likely caused by the packages installed from the external repositories. Unfortunately, Ubuntu is not responsible for maintaining it and cannot ensure safe upgrade process with third-party software installed.

Removing said repository and running:
  aptitude update && aptitude dist-upgrade
should help, but it will most likely cause (or propose) removing the third-party software. The only thing I can suggest it installing it again after the upgrade, since the ubuntu-desktop package is a meta-package needed only during the upgrades.

Thanks again for your cooperation.