Login.Aspxで、次の変更を行いました。
protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            Master.FindControl("CAMenu").Visible = false;
            Master.FindControl("loginStatus").Visible = false;
        }
        catch (Exception ex)
        {
            new Logger().Log("ShortCom.Login.btnLogin_Click(object sender, EventArgs e)", ex.Message);
            Response.Redirect("~/Error.aspx");
        }
    }
    protected void LoadMessageBox(string MessageID)
    {
        try
        {
            messages = new GUIMessages();
            popupExtend = new ModalPopupExtender();
            lbMessage = (Label)Master.FindControl("label5");
            lbMessage.Text = messages.GetGUIMessage(GUIModule.Login, MessageID);
            popupExtend = (ModalPopupExtender)Master.FindControl("popupExtender");
            popupExtend.Show();
        }
        catch (Exception ex)
        {
            new Logger().Log("ShortCom.Login.LoadMessageBox(string MessageID)", ex.Message);
            Response.Redirect("~/Error.aspx");
        }
    }
    protected void btnLogin_Click(object sender, EventArgs e)
    {
        try
        {
            string userName = txtUsername.Text;
            string password = txtPassword.Text;
            if (userName == string.Empty && password == string.Empty)
            {
                LoadMessageBox("5");
                txtUsername.Focus();
                return;
            }
            if (userName == string.Empty)
            {
                LoadMessageBox("1");
                txtUsername.Focus();
                return;
            }
            else if (password == string.Empty)
            {
                LoadMessageBox("3");
                txtPassword.Focus();
                return;
            }
            User user = new User(userName);
            DataTable tab = new DataTable();
            tab = user.GetUserDetails(userName);
            string firstName = string.Empty;
            string userPassword = string.Empty;
            string RoleID = string.Empty;
            string userID = string.Empty;
            Session["UserName"] = userName;
            if (tab.Rows.Count == 0)
            {
                LoadMessageBox("6");
                txtPassword.Text = string.Empty;
                txtUsername.Text = string.Empty;
                txtUsername.Focus();
                return;
            }
            if (tab.Rows.Count == 1)
            {
                userID = tab.Rows[0][0].ToString();
                firstName = tab.Rows[0][1].ToString();
                userPassword = tab.Rows[0][2].ToString();
                RoleID = tab.Rows[0][3].ToString();
                Session["UserID"] = userID;
            }
                if (userPassword == password)
                {
                    Response.Redirect("~/Default.aspx");
                }
                else
                {
                    LoadMessageBox("4");
                    txtPassword.Focus();
                    return;
                }
        }
        catch (Exception ex)
        {
            new Logger().Log("ShortCom.Login.btnLogin_Click(object sender, EventArgs e)", ex.Message);
            Response.Redirect("~/Error.aspx");
        }
    }
そしてSite.Masterで:-
protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            Page.Response.Cache.SetCacheability(HttpCacheability.NoCache);
            if (Page.Title != "Login")
            {
                if (Session.Count == 0 || Session["Username"] == null)
                    Response.Redirect("~/Login.aspx", true);
                CheckRole();
            }
            lblDateTime.Text = "";
        }
        catch (Exception ex)
        {
            new Logger().Log("ShortCom.SiteMaster.Page_Load()(object sender, EventArgs e)", ex.Message);
            Response.Redirect("~/Error.aspx");
        }
      }
    public void CheckRole()
    {
        try
        {
            if (System.Web.HttpContext.Current.Session.Count > 0)
            {
                string firstName = string.Empty;
                // string userPassword = string.Empty;
                string RoleID = string.Empty;
                Common common = new Common();
                DataTable tab = new DataTable();
                string userName = (string)Session["UserName"];
                User user = new User(userName);
                tab = user.GetUserDetails(userName);
                if (tab.Rows.Count == 1)
                {
                    firstName = tab.Rows[0][1].ToString();
                    RoleID = tab.Rows[0][3].ToString();
                }
                if (RoleID != "1")
                {
                    int count = CAMenu.Items.Count;
                    if (count == 5)
                    {
                        for (int menuCount = 3; menuCount > 0; menuCount--)
                        {
                            string text = CAMenu.Items[menuCount - 1].Text;
                            CAMenu.Items.RemoveAt(menuCount - 1);
                        }
                    }
                    lbLoginMessage.Text = "Welcome," + " " + firstName;
                    loginStatus.Visible = true;
                }
                else
                {
                    lbLoginMessage.Text = "Welcome," + " " + firstName;
                    loginStatus.Visible = true;
                }
            }
            else
            {
                Session.Abandon();
                Response.Redirect("~/Login.aspx", true);
            }
        }
        catch (Exception ex)
        {
            new Logger().Log("ShortCom.SiteMaster.CheckRole()", ex.Message);
            Response.Redirect("~/Error.aspx");
        }
    }