While upgrading to 14.04 using sudo do-release-upgrade my network connection was interrupted and several packages failed to upgrade.

Running do-release-upgrade again reported No new releases found but apt-get listed many packages "kept back", so it seemed the system was in an intermediate state. I ran apt-get dist-upgrade which installed those packages again, then apt-get autoremove after which apt seemed happy.

However, after restarting I cannot log in at the GUI prompt. After entering my password the error "Failed to start session" appears in red. I am still able to log in from the command prompt, but I don't know what to do to solve or debug the GUI log in failure.