回发后表单不呈现更新的标签

Question

我一直遇到按钮提交回发的问题，因为按钮完成后网站认为它是完全刷新，这使我无法显示登录消息或错误的任何失败，

我似乎无法确定为什么会发生这种情况，并且已经查看了页面循环，但无法将其放在上面。

这是我在页面后面的所有代码

protected void Page_Init(object sender, EventArgs e)
    {
        Session["user"]     = "";
        Session["domain"]   = "";
        Session["manager"]  = "";
    }
    protected void Page_Load(object sender, EventArgs e)
    {
        if (Page.IsPostBack)
            lblError.Text = "This was a post back.";
        else
            lblError.Text = "This was NOT a post back.";
    }
    protected void btnLogin_Click(object sender, EventArgs e)
    {
        //Allows the variable to be used through out the app
        Session["user"]     = username;
        Session["domain"]   = domain;
        Session["manager"]  = null;

        if (true == authenticateUser(Session["domain"].ToString(), Session["user"].ToString(), password))
            Response.Redirect(Response.ApplyAppPathModifier("Update.aspx"));
    }

和前面

<form id="form1" runat="server" style="Width:19%;" class="pure-form pure-   form-stacked">
    <asp:ScriptManager runat="server"></asp:ScriptManager>
    <asp:Label ID="lblName"  runat="server" Text="Name"></asp:Label>
    <br />
        <asp:TextBox ID="txtLoginID" Width="95%" Style="display:inline-block;" runat="server"></asp:TextBox> 
     </div>
    <br/>
    <asp:Label ID="lblpwd" runat="server" Text="Password"></asp:Label>
    <asp:TextBox ID="txtPassword" Width="95%" TextMode="Password" runat="server"></asp:TextBox>
    <asp:Label ID="lbldmn" runat="server" Text="Domain"></asp:Label>
    <asp:DropDownList ID="ddlDomain" Width="95%" runat="server">
        <asp:ListItem>hdnl.it</asp:ListItem>
        <asp:ListItem>yodel.net</asp:ListItem>
    </asp:DropDownList>
    <asp:Label ID="lblError" runat="server" ForeColor="Red" ></asp:Label>
    <br />

    <asp:UpdatePanel ID="UpdatePanel1" runat="server"> 
        <ContentTemplate> 
            <asp:Button ID="btnLogin" runat="server" Text="Login" Width="95%" OnClick="btnLogin_Click" 
                CssClass="button-success pure-button" />
        </ContentTemplate> 
    </asp:UpdatePanel> 
    <asp:UpdateProgress ID="UpdateProgress1"  runat="server" 
    AssociatedUpdatePanelID="UpdatePanel1"> 
        <ProgressTemplate> 
            <div class="overlay"></div>
            <div class="modal">
                <h2>Please Wait.....</h2>
                <img alt="Loading..." src="/Images/loading.gif" />
            </div>
        </ProgressTemplate>
    </asp:UpdateProgress> 
</form>