Alan,
I don't think it's project specific since I have been able to re-create in a new file.
In the new file, I dragged in a ttm and a dxf file - then saved the tbc file.
Open the export dialog box and pick on "Machine Job Site Design exporter" - which causes the issue.
I have to open task manager, end TBC / delete the lock file. Go back to TBC, open the file and open export
dialog, which always seems to work on the second try.