第一次在项目中用AJAX就受到严重打击 UpdatePanel+Timer

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Timer ID="Timer1" runat="server" Interval="60000" OnTick="Timer1_Tick">
                </asp:Timer>
   <div align="left" style="FONT-SIZE: 10pt; COLOR: #ffffff">
        &nbsp;
        结束时间：<asp:Label ID="Label_1" runat="server" Font-Bold="False"></asp:Label>
        当前时间：<asp:Label ID="Label_2" runat="server" Font-Bold="False"></asp:Label>
   </div> 
           </ContentTemplate>
</asp:UpdatePanel>

为什么页面不时会弹下面的提示框出来。。。。。这么简单的控件都这样，受打击。

---------------------------
Microsoft Internet Explorer
---------------------------
Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 12031
---------------------------
确定  
---------------------------