I am trying to convert a .xls file to an .xlsx file on the server-side using Microsoft.Office.Interop.Excel.Workbook class as follows:
Microsoft.Office.Interop.Excel.Workbook
workBook
Try to add Thread.Sleep Method, such as Thread.Sleep(2000) for 2 seconds after workBook.open and workBook.SaveAs two methods. If your Excel file has a lot of formats, try to extend few more seconds.