It could a problem with all the big meshes but first have you tried saving the map and then closing your Feral Heart and restarting again before trying to export? As for those custom meshes you used make sure that you put them into their own object groups. Don't just include them along with the default groups.
Every file is downloaded as it came, and I do not often put meshes manually downloaded into default groups.
I've made several maps before, and they all usually export just fine.
I've discovered; The map creates a .fhm for the map in exports, as it should, but not every file is put into .fho form.
I have tried saving then restarting; No good.