2 ideas based on a search in the 'archives'
Are you exporting to Excel as a V9 export or a V8 export? (Search the forum using 'Summary Excel Export' for a list of possibly related challenges!)
V9 has additional features but sometimes some people have found some problems exporting as V9.
The other oddity has been something obscure related to the current default printer on the hosting PC ... reasons seem to be unidentified but changing the default printer has worked for some people with problems that sound kind of similar to yours.
If neither of those work in your case we will have to continue to look for another answer.
I've spent the morning looking for a reason why all of sudden all of my exports (tables and summaries) were getting 'Unable to open' messages from Excel 2003.
I had figured out I could turn off the excel formatting and at least open the file, but then had to reformat columns/headers and such.
Checked again after lunch and found Grants response. Among all the service packs, I installed last week I Never would have thought it would be related to my default printer. Sure enough, I also changed printers last week, a quick change of defaults and I'm back in business.