Why does my webpage redirect to another page automatically after login?

Hi All,
i installed our website E on our server A. it works fine on that server. but when i tried to access it from my machine (note: not server A ), i had the following issues:
1. load the login page --> works fine
2. key in my credential and click LogIn button--> it automatically jumps to the default page of our public website O

what i expected is: after login, it should go to page C under website E.
Note: website E is the subwebsite of website O

in Login page:
protected void bLogin_Click(object sender, ImageClickEventArgs e)
    {
        if (!Page.IsValid)
            return;

        var user = WebUser.getByEmail(tbUsername.Text.Trim());
        if (user != null)
        {
            if (user.VerifyPassword(tbPassword.Text, true))
            {
                // store the user object so we can reference it on pages.
                Session["USER"] = user;

                // prevent multiple login by the same user (see Global.asax)
                Application["SessionID " + user.Id] = this.Session.SessionID;
               
//this is where the login page should go to. it goes to next page C when running on server A locally. but jumps to public website O when testing it on my machine                                 System.Web.Security.FormsAuthentication.RedirectFromLoginPage(user.Id.ToString(), false);
               
            }
        }
    }
******************************
web.config:

<authentication mode="Forms">
      <forms loginUrl="~/login.aspx" timeout="20" defaultUrl="~/Default.aspx" />
    </authentication>
    <authorization>
      <deny users="?" />
      <allow users="*" />
    </authorization>

**********************

Can anyone please point me to the right direction?

thanks heaps