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QISSingleSignOn fails to login existing users or create new users when applied to MW 1.9.x or 1.8.x. However, it does so 
flawless with 1.7.x and 1.6.

QISSingleSignOn utilizes AuthPlugin and $wgHooks['AutoAuthenticate'].

*** Bug 9389 has been marked as a duplicate of this bug. ***

One Solution: Comment out or remove the following lines in function QISSingleSignOnAutoAuthenticate 
in /extensions/QISSingleSignOn.php

#        if ( $user != null) {
                // user is authenticated (by another hook)
#                return true;
#        }

QISSingleSignOn is not part of SVN -> unable to apply any patch -> closing this bug as INVALID.