Comment 7 for bug 1354825

Addition to my previous post. There is another file in the home directory, namely ~/.cache/update-manager-core/meta-release-lts that suffered the same problem that the file in /var/lib mentioned in my previous post.
It was neccesary to delete also this file so that the update is offered not only in text mode (do-release-upgrade) but also in the graphical interface of update-manager.