Message: Unable to read file.. while opening grid view exported to excel

Protected Sub ExportToExcel(sender As Object, e As EventArgs)
        Response.Clear()
        Response.Buffer = True
        Response.AddHeader("content-disposition", "attachment;filename=Scope.xls")
        'Response.AddHeader("content-disposition", "attachment;filename=Scope.xlsx")
        Response.Charset = ""
        Response.ContentType = "application/vnd.ms-excel"
        Using sw As New StringWriter()
            Dim hw As New HtmlTextWriter(sw)

            'To Export all pages
            gdScope.AllowPaging = False

            Me.btnShow_Click(Me, e)

            gdScope.HeaderRow.BackColor = Color.White
            For Each cell As TableCell In gdScope.HeaderRow.Cells
                cell.BackColor = gdScope.HeaderStyle.BackColor
            Next
            For Each row As GridViewRow In gdScope.Rows
                row.BackColor = Color.White
                For Each cell As TableCell In row.Cells
                    If row.RowIndex Mod 2 = 0 Then
                        cell.BackColor = gdScope.AlternatingRowStyle.BackColor
                    Else
                        cell.BackColor = gdScope.RowStyle.BackColor
                    End If
                    cell.CssClass = "textmode"
                Next
            Next

            gdScope.RenderControl(hw)
            'style to format numbers to string
            Dim style As String = "<style> .textmode { } </style>"
            Response.Write(style)
            Response.Output.Write(sw.ToString())
            Response.Flush()
            Response.[End]()
        End Using
    End Sub


    Public Overrides Sub VerifyRenderingInServerForm(control As Control)
        ' Verifies that the control is rendered
    End Sub

<connectionStrings>
   <add name ="Excel03ConString" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source={0};Extended Properties='Excel 8.0;HDR={1}'"/>
   <add name ="Excel07ConString" connectionString="Provider=Microsoft.ACE.OLEDB.12.0;Data Source={0};Extended Properties='Excel 8.0;HDR={1}'"/>
 </connectionStrings >