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In an attempt to fix the problem, I uninstalled lightdm and gdm is now the default. But I experience the same problem with gdm. When I try to reinstall lighten, I receive:

WARNING: The following packages cannot be authenticated! 
Install these packages without verification [y/N]? y
Err raring-updatea/main lighten amd64 1.6.0-ubuntu3
Something wicked happen ed resolving '' (-11 - System error)
Failed to fetch So me thing wicked happened resolving '' (-11 - System error)
E: Unable to fetch some archives, maybe run apt-get update or try with --fix missing?
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There is error in URL in your /etc/apt/sources.list:

  1. Run sudo nano /etc/apt/sources.list,

  2. Find text raring-updatea/main

    And replace it with raring-updates/main

  3. Run

     sudo apt-get update
     sudo apt-get upgrade
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When I turned on my PC, all issues were resolved. Weird. – JasonO Sep 8 '13 at 17:15

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