I noticed that I didn't have the ubuntu-desktop package (though I had all its dependencies and I believe I didn't uninstalled it). I installed that package and then run again the update-manager.
This time I got another error: "Could not calculate the upgrade. A unresolvable problem occured while calculating the upgrade. Please report this as a bug.". The application exited cleanly after than, reverting the changes made to sources.list.
I will try a manual dist-upgrade, to test if this is really a problem with apt-get.
I noticed that I didn't have the ubuntu-desktop package (though I had all its dependencies and I believe I didn't uninstalled it). I installed that package and then run again the update-manager.
This time I got another error: "Could not calculate the upgrade. A unresolvable problem occured while calculating the upgrade. Please report this as a bug.". The application exited cleanly after than, reverting the changes made to sources.list.
I will try a manual dist-upgrade, to test if this is really a problem with apt-get.