It sounds like the export is including something in that field that Excel does not like. Maybe something that makes up part of a control sequence perhaps, or something that needs to be stripped from the original data.
Have you tried hiding but still exporting the field (or is that what you did anyway)? Does Excel still complain?
It sounds like that field may contain some sort of control character(s) that Excel does not much like.
You could try exporting just that field to a file and then running the EXAMINE.EXE utility on it to see what characters if contains. If you are only expecting alpha numeric and other values appear (other than the usual formatting characters and combinations) there may be something that Excel interprets and does not like.
What to do about it is another matter ...
Of course it is always possible that this will come up with nothing obvious or even useful but it should not take long to try it out.
Thank you, Grant Perkins.
The problem cropped up on some other exported summaries, so that I couldn't open them in Excel 2003. It was critical to find a work around (maybe even going back to V8) so I called Tech Support. They knew how to fix it right away.
Go to Options, Export and Clipboard, and select Excel 97+(Monarch V8). There is another option for Monarch V9, but unless you are running Excel 2007, you should probably set this to Monarch V8.
This fixed the problem right away.