Comment 2 for bug 574766

Hi Henry. Thanks for your bug report. This is most likely because policykit is not running. Could you please verify that 'polkit-gnome-authentication' is not listed by system > administration > system monitor? This could be because policykit has crashed (in which case logging out and logging in again should make software-center work again). Please continue to report any bugs you find in software-center. Thanks.