I upgraded from Ubuntu Lucid Lynx to Maverick Meerkat with the following commands:

sudo apt-get update && sudo apt-get upgrade
sudo apt-get install update-manager-core
sudo vi /etc/update-manager/release-upgrades

and changed the last line to Prompt=normal

sudo do-release-upgrade -d

This upgrade was OK.

I decided to repeat the same steps and to upgrade Maverick Meerkat to Natty Narwhal. It ended with this message:

Building data structures... Done

Calculating the changes

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade: Can not mark 'xubuntu-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.

Restoring original system state

Aborting Reading package lists... Done Building dependency tree
Reading state information... Done Building data structures... Done
=== Command detached from window (Mon Nov 21 09:37:21 2011) ===
=== Command terminated with exit status 1 (Mon Nov 21 09:37:21 2011) ===

How can I correct it?