I decided to update to Oneiric using the update manager. At some point, I got this message:

The upgrade has aborted. Your system could be in an unusable state. A recovery will run now (dpkg --configure -a).

Then, later on:

The upgrade has completed but there were errors during the upgrade process.

What can I do?

EDIT: Update manager offers to perform a 'Partial Upgrade'. "865 updates have been selected. The updates have already been downloaded, but not installed". It still gives the option to upgrade to 11.10