An unresolvable problem occurred while calculating the upgrade:

Can not mark 'ubuntu-desktop' for upgrade

 

 This can be caused by:

 * Upgrading to a pre-release version of Ubuntu

 * Running the current pre-release version of Ubuntu

 * Unofficial software packages not provided by Ubuntu

 

If none of this applies, then please report this bug against the 'update-manager' package and include the files in /var/log/dist-upgrade/ in the bug report.