This could be a corrupted Excel file or an issue with .Net.
Can you create a new Excel file and add data to it? If so, is this a viable fix, or maybe you can obtain a tool to analyze and fix any Excel issues.
Or it could be a .net issue. Monarch uses many Microsoft components to export data and they can get a bit out of shape sometimes. Check with your IT department first to make sure they have no objections, but I’d install .Net 3.5 from http://www.microsoft.com/NET/[/URL]
As a short term workaround, you may be able to export the data using an older version of the export engine. Launch Monarch, then go into the Options->Export and clipboard menu and work back through the list. You will lose some formatting enhancements and speed, and even the ability to add data to an existing file if you go back to far. So I’d suggest making a back up of the Excel file so you don’t accidently overwrite it.