1827176 - Error: “Excel Found unreadable content in 'File name .xlsx'" when open the .xlsx file, generated from Webi Intelligence in BOXI 3.1 SP6

Using BOXI 3.1 SP6, when a webi report is saved in Excel 2007 format (new feature with SP6), this generated excel file fails to open with the following error-
"Error: “Excel Found unreadable content in 'File name .xlsx'. Do you want to recover the content of this workbook? If you trust the source of this workbook, click Yes” when open the Excel file generated from Webi."

If click on ‘Yes’ for the above message then get the another error:
“Excel cannot open the file ‘filename.xlsx’ because file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file”

BusinessObjects Enterprise XI 3.1 SP6

KBA , BI-RA-WBI , Web Intelligence , Problem