CR for VS2010 produces unreadable results when exporting to RTF or XLSX

In my VS2010 (w/ CR2010) windows application, I provide the possibility to export a report in a given format. If the FormatType is PDF, Text or XLS (Excel 97-2003), the output is perfectly readable by the associated program (Acrobat Reader, Notepad or Excel 2003).

However, if I use a (editable)RTF, WordForWindows or Excel Workbook (XLSX) output format, Microsoft Word (2007 / 20010, Excel 207 / 2010) complains about a damaged file.

What am I doing wrong?

Regards,

Adri