I am the administrator of a prediction community but I cannot access the admin area although I am logged in.
You have to be logged in with the administrator's e-mail address. Try to log in with another e-mail address. If you don't remember the password, use the Password forgotten feature.
By the way: If you succeed to log in with a second e-mail address, you should go to "My profile" and change your e-mail address, so that you only need one login and have all roles in one login. Then you don't need to log out and log back in.