На Login.aspx
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim _login As New MAPLibrary.Login(Application("ConnectionString"))
        If _login.Login(Me.UserName.Text, Me.Password.Text) Then            
            Session("UserCode") = _login.LoginCode
            Dim _authTicket As New System.Web.Security.FormsAuthenticationTicket( 1 , Me.UserName.Text, DateTime.Now, DateTime.Now.AddMinutes( 20 ), True, _login.LoginType)

            Dim _encryptedTicket As String = FormsAuthentication.Encrypt(_authTicket)
            Dim _authCookie As New HttpCookie(FormsAuthentication.FormsCookieName, _encryptedTicket)
            HttpContext.Current.Response.Cookies.Add(_authCookie)

            Dim _id As New System.Web.Security.FormsIdentity(_authTicket)
            Dim _roles() As String = Split(_login.LoginType)
            HttpContext.Current.User = New System.Security.Principal.GenericPrincipal(_id, _roles)

            'If HttpContext.Current.User.IsInRole("Клиент") Then Response.Redirect("Customer/Default.aspx") ' Эта строчка отрабатывает

            Response.Redirect(FormsAuthentication.GetRedirectUrl(Me.UserName.Text, False))
        Else
...
        End If    
    End Sub

Default.aspx - Ни одно их этих условий (Роль = Клиент должно быть) не срабатывает
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
        If Session("UserCode") = "" Then Response.Redirect("Login.aspx")
        If HttpContext.Current.User.IsInRole("Клиент") Then Response.Redirect("Customer/Default.aspx")
        If HttpContext.Current.User.IsInRole("Офис") Then Response.Redirect("Region/Default.aspx")
        
    End Sub

Dim _id As New System.Web.Security.FormsIdentity(_authTicket)
Dim _roles() As String = Split(_login.LoginType)
HttpContext.Current.User = New System.Security.Principal.GenericPrincipal(_id, _roles)