ASP.NET

Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed.

<asp:UpdatePanel ID="Upnl1" runat="server">
<Triggers>
 <asp:PostBackTrigger ControlID="btnAddLineItems" />
</Triggers>
 <ContentTemplate>
    <div>
    </div>
    <asp:panel>
    <table>
        txt1
        txt2
        ddl1
        ddl2
        File upload
        Button Add new Items
    </table>
    </asp:panel>

    <asp:updatepanel>
        <ContentTemplate>
        <Gridview>
        <Boundfield 1=txt1>
        <Boundfield 2=txt2>
        <Boundfield 3=ddl1>
        <Boundfield 4=ddl3>
        <Templatefield Link button="Download">
        </Gridview>
        </ContentTemplate>
    <asp:updatepanel>

 </ContentTemplate>
</asp:UpdatePanel>

Dim filePath As String = CType(sender, LinkButton).CommandArgument
       Response.ContentType = ContentType
       Response.AppendHeader("Content-Disposition", ("attachment; filename=" + Path.GetFileName(filePath)))
       Response.WriteFile(filePath)
       Response.End()