xls file that only Excel 2003 can open

Hi guys, 

We have a problem with a xls file generated by a program made by a colleague (PHP program / pear library "Spreadsheet_Excel_Writer").

This very simple worksheet has few rows and coloums but only Excel 2003 can open it without problems.

Excel 2007/2010/2013 can't open the file. This is the error message: "Excel has encountered a problem with a piece of content".

If I open the file with Excel 2003 and I save it, just save as xls, then Excel 2007/2010/2013 can open the file.

The only thing I have noticed is that, when I do "Save as" in Excel 2003, the file type is "Excel 5.0/95".

Thanks in advance

Diego