After hearing about some of the features in 12.10, I decided to upgrade my Ubuntu from 12.04 LTS to the newer 12.10. However, when I try to upgrade it, I start the update manager, like always, and I start the distro upgrade, just like I do every time there's a new version, but at the end of the "Setting new software channels" stage, it fails, saying:

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade:
E: Unable to correct problems, you have held broken packages.

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 using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

What should I do? I understand that I have some sort of "broken" package on my system, but how do I identify and remove the problem?

Note: At the start of the "Setting new software channels" stage, I was presented with this:

Third party sources disabled

Some third party entries in your sources.list were disabled. You can re-enable them after the upgrade with the 'software-properties' tool or your package manager.