Go to your "terrains" folder and delete the folder that has the map name you created. After that, re-download the map.
I am not really experienced in this area, but I hope this solution will work out for you!
Have you downloaded any other meshes and saved them into the default object groups? This could very much damage those default groups and mess with your map's objects.
If not, I'd try manually moving the .obj files you used into your exports folder and having it replace them when prompted.