Particles, or where items are placed is an issue that you might not think of. If you have any mesh in the default folder, that can usually cause some problems with the exporting.
Also make sure that your height map and files are in the right places, and that when you export the map, or try to, it ACTUALLY shows up in the exports. Check which files everything is in.
If you have some of your own meshes, you might want to look over what you did to those, because something could be corrupting your game in that sense.