We are afraid, you are using very old version and we only fix the issue in most recent version only.
However, please try this code and check if it makes any difference and works for you.
C#
//Save file and send to client browser using selected format
if (yourFileFormat == "XLS")
{
workbook.Save(HttpContext.Current.Response, "output.xls", ContentDisposition.Attachment, new XlsSaveOptions(SaveFormat.Excel97To2003));
}
else
{
workbook.Save(HttpContext.Current.Response, "output.xlsx", ContentDisposition.Attachment, new OoxmlSaveOptions(SaveFormat.Xlsx));
}
HttpContext.Current.Response.End();