Error while using Update Panel

Response does not work with updatepanels. Let me know what youa re trying to do.

If you are trying to download file, I would like you to inform that upload and download files require full postback and does not work with UpdatePanels

here is the code , i am exporting grid to excel

Private Sub ExportToExcel(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExportEXL.Click
        'lblError.Text = ""
        If (GridMails.Rows.Count <> 0) Then
            Dim fileName As String
            fileName = System.Guid.NewGuid.ToString()

            Session("TempSession") = Session("GridMails")
            Dim gridhtml As New System.Web.UI.WebControls.GridView()
            Dim dt = TryCast(Session("GridMails"), DataTable)
            Dim newDt = dt.Copy
            newDt.Columns.Remove("DBMonth")
            newDt.Columns.Remove("DBYear")
            newDt.Columns.Remove("MailID")
            newDt.Columns.Remove("AddedToDocit")
            gridhtml.DataSource = newDt
            gridhtml.DataBind()
            'formating
            gridhtml.HeaderRow.BackColor = System.Drawing.ColorTranslator.FromHtml("#CCCCCC")
            Dim sw As New StringWriter()
            Dim htw As New System.Web.UI.HtmlTextWriter(sw)

            gridhtml.RenderControl(htw)

            Response.Clear()
            Response.Charset = ""
            Response.Buffer = True
            Response.ContentType = "application/vnd.ms-excel"
            Response.AddHeader("content-disposition", "attachment;filename=" & fileName & ".xls")
            Response.Write(sw.ToString())
            Response.End()
        Else
            lblError.Text = "No Data to Export."

        End If
    End Sub

This will not work with AsycPostBackTrigger as it is download code.

You will need to change the trigger for the download button to PostBackTrigger so that full postback occurs

I have changed the trigger and now its working

Thanks