Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade:
The package 'xubuntu-desktop' is marked for removal but it is in the removal blacklist.

 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

This is most likely a transient problem, please try again later.