asp.net login control logged in event - keep getting return URL

I am using standard asp.net membership and roles that works fine.
Once logged in I am using the "logged in event" to authenticate to a third party forum application that also seems to work, ie, it does authenticate the user on the forum application.  

However once authenticated on the forum I cannot get past the login screen, I keep getting the returnURL back to login page.
If I take out the forum authentication code it works fine.

Here is a sample of the code I use to authenticate once logged in

                    // create an instance of the ForumApp.ForumApp.Components.User object
                    // from the user identity within the ForumApp_Users table
                    ForumApp.ForumApp.Components.User User = new ForumApp.ForumApp.Components.User(UserID);
                    // and log the ForumApp.ForumApp.Components.User object into the forums
                    User.Authenticate(false, false);
                    ForumApp.Common.Authentication.Authentication.Login(User, true);


However I cannot get past the login screen,  if I take it out it works fine.