Forms authentication on IIS 7

Hi,

One customer set up an IIS 7 on a Windows 2008 machine. Forms Authentication works fine on my local IIS 6.0 and also on a shared server I used for testing.  On the new machine there is no way no access the password protected area despite a succesful login.
Is there any settings I should ask my customer to check out on the server ? Or is there something different for IIS 7 I should see in my web.config ? 
...