File size is pretty important in Navisworks. We export out our NWD and our field guys use that with BIM 360 Glue to view the model in the field. However Glue apparently has a ±50MB file size limit. As such I've had to be very picky about what I include in my models. 

Back to the issue at hand...

Have you tried to reinstall the Navisworks Exporter? Go to the control panel > Programs > search for Navis > find the relevant version of Navis exporter > Uninstall/Change > Repair or Reinstall > Reinstall

If it doesn't work rinse and repeat with the full Navisworks.