[ubuntu] Update manager and software center not working

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http://ubuntuforums.org – Hello all, I am beginning to use Ubuntu 12.02, and I have problems with the Update Manager and the software center (I guess something must be wrong with my repository list...). The update Manager freezes when I click on install updates (and gives me lots of suggestions even though I regularly run apt-get upgrade and apt-get update), and the software center just won't start (it just shows a grey window). I have been trying to uninstall them both from terminal, update and upgrade, and re-install them, but it did not change anything. Any help would be welcome ! (Hardware)