Courtesy of Doug Curtis in OIT:
People are receiving a 500 error when using the "GT Logout" link on a Drupal page. It looks like the problem is that the Drupal CAS module is appending the "service=" query string to the end of the CAS logout URL. The GT CAS server will accept "url=" query string but it doesn't accept the "service=" query string. Previously, the CAS service would quietly ignore appended query strings it didn't recognize but that isn't currently the case.
The way to fix this is to leave the "Logout destination" field blank in the phpCAS module's settings. The "Logout destination" is supposed to be used to specify a URL to redirect the user to after the user has been logged out. It is NOT supposed to be the URL for logging out of CAS. By leaving the field blank, the application will be logged out of your Drupal site, but the user will still be authenticated in CAS.