I'm not sure I fully understand what you are asking. We currently use Excel 2007 and have no issues exporting portelts. When you click on the "Export to Excel" link at the bottom of the portlet and then click "Yes" to open with Excel, a blank Internet Explorer window comes up with a warning that I have attached. Clicking yes only verifies that you trust what is opening because the file extension is different. Hope that helps.
Let me clear what I would like to accomplish. We are using Excel 2007 on our PCs. When "Export to Excel" link is hit on Request Search Results Page or on List Portlets, we are asked to open/save it with .xls format. Then the bug you have posted occurs and the only solution is to save it in .xlsx format. Is there a workaround to maybe change the codes in ListPortletExportToExcel.jsp or other related files so that it's saved directly in Excel 2007 format?