I finally upgraded by system. The problem was in the file
/etc/update-manager/release-upgrades.
I changed the line
Prompt=lts
to
Prompt=normal
and the upgrade succeeded. I was then able to upgrade to 14.04, so I'm up to date.
Thanks to all who helped.
I'd like to change the status of the tread to "Solved", but I don't know how.
Bookmarks