The "bug" was due to Debian "Sid". I do not why I have added this unstable package, but I removed it, and everything seems OK now. At least the automatic update has been able to finish without any error message any more.
** Changed in: update-manager (Ubuntu) Status: New => Fix Released -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1267179 Title: impossible to install "ubuntu-desktop" (in French) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/1267179/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs