I was surfing accross SDN today and there I found at the known issues part for Sitecore 5.1.1 the following note:
Extranet security changes require IIS Restart
Access right assignments you make for the Extranet Users and Roles won’t take effect until IIS is restarted. This issue can be fixed by installing the Extranet Security Issue patch.
Now I finally understand why my all my script didn’t work in the first place but worked after I placed it between the using(new SecurityDisabler())-bracket.
When these problems occur at your installation. Login at SDN and download the patch