When I wanted to upgrade from Ubuntu 13.10 to 14.04 the update manager told me that there was an unresolvable problem with calculating the upgrade. In order to see more details I performed the following on the command line:

apt-get update
apt-get upgrade
apt-get dist-upgrade
apt-get install update-manager-core
do-release-upgrade

Now do-release-upgrade tells me that no newer version is available but the "about" box in the main menu tells me I am still on 13.10. I think the latter is right, as for instance in the "system settings" there is no way to select local menus, which is present in 14.04 (I tried with a DVD, and there it is).

Maybe the problem is related to the fact that "apt-get update" cannot get two resources called "saucy/InRelease" and "saucy/main/binary-i386/Packages" from the mirror (mirror.informatik.uni-mannheim.de; 404 not found). I would like to use a different mirror; how can I do that?

Any help appreciated, thanks in advance.