Comment 5 for bug 1354825

I had the same problem, it was NOT related to the prompt value. The only way that I managed to fix it was to uninstall and purge update-manager-core (which will remove update-notifier-common) and then reinstall it. After that upgrade proceeded normally. (I did manually delete the /var/lib/update-manager and /var/lib/update-notifier directories as the purge command refused to).