I think it must have had something to do with the sort order because I found all the records and did not sort them, then found the same set of records (also no sorting them) and now it exports fine.
It's also possible your file (or at least an index) may be damaged.
You might try importing your data into a clone of the file (that rebuilds all indexes) or even running a recover to see if it reports finding any damage in the file. If recover reports problems, best practice is to replace your file with a backup copy that is undamaged.
Thanks. I am working on that, too.