I upgraded an existing 8.10 Ubuntu installation to 9.04 last week, and everything appeared to run fine initially. However now on boot I get a dialog box which says 'Not all updates can be installed'. Hitting the 'Partial Upgrade' button the list includes
If I then go with install the upgrade the dialog box disappears (after the permission to proceed password prompt) and nothing further happens until the next reboot, when I'm back to the partial upgrade prompt again.
Where should I look to find out what's going wrong and fix this?