I made a silly mistake. Following your guidelines I created a user group "Accepted Cookies".
After that I started to choose the new access level I just created in the "Accepted Cookies Access Level" setting.
Unfortunately I think I also put the authentification plugin into the "Accepted Cookies Access Level". If I try to login I only get the answer "JAuthentication: :__construct: Could not load authentication libraries."
Could you please help me? I tried to change the Access Level to public again. But unfortunately I am unable to find the right table in phpmyadmin.