Hi Masai,
geht der der Zugriff per SSL (ohne FormsBasedAuth) ?
Schau mal hier, vielleicht hilfts:
834637 The „Enable Forms Based Authentication“ check box is unavailable in
http://support.microsoft.com/?id=834637
830827 How to Use Forms Based Authentication with Outlook Web Access Clients in
http://support.microsoft.com/?id=830827
Gruß
Nicole