Server error in Asp.Net web app, Office / Excel Interop Exception from HRESULT: 0x800A03EC

IF you should happen to get an error like the following when saving an excel (or other office document I guess) workbook, try opening Excel/Word etc manually and see if the product needs activation! I came across this error earlier today after the project was working 100% and it turned out I just needed to activate the product!

Thanks Microsoft, you just wasted an hour of my time today trying to sort it out!

Server Error in ‘/’ Application.

Exception from HRESULT: 0x800A03EC

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.Runtime.InteropServices.COMException: Exception from HRESULT: 0x800A03EC Source Error: Line 496: Line 497: // create the new file Line 498: xlWorkBook.SaveAs(strFileId, Excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, Excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue); Line 499: xlWorkBook.Close(true, misValue, misValue); Line 500: