So it must be the database, and it seems you have no backup. It's been a very long time since I've used gfix. When you used it, was there anything that suggested another try might help?
Does the error happen regardless of which and how many records are selected for export? If it occurs only with one or a few corrupt records, maybe you can find and delete them. Or avoid them and export the rest. The purpose of doing that would be to export all that you can, and then import that into a new database.