Check the file in DFDM. See if any record has an odd-looking key. Delete that record. If it's a line record, you should be able to go back and re-enter. If it's a header record, it may not work well.
Or you could try rebuilding sort/key files.
Here's an ancient link: http://sagecity.na.sage.com/support_communities/sage100_erp/f/92/t/33385