The dist upgrade is placing the 'independent' repo on all of the source URIs in sources.list. When did 'independent' repos get into the dist channels? Oh, wait... http://us.archive.ubuntu.com/ubuntu/dists/precise/ . They aren't. :) just the regulars. Easy fix.
open up your sources.list in etc/apt as root, delete 'independent' from every dist url and save. Attached is the corrected sources.list for reference, ** Attachment added: "corrected the distro entries" https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/947296/+attachment/3137795/+files/sources.list -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/947296 Title: Update-manager stopped updating To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/947296/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
