Can you please provide us with the exception you get from the User profile module ?
if you have AD, Windows authentication and ASP.NET impersonation turned on then your users are automatically authenticated. In that case you need to call the following method in some Web site entry point:
UserRepository repository = UserRepository.GetInstance();
AspnetUsersEntity authenticatedUser = repository.GetUser(
//Your error custom message
If you need further help let us know.