Two possibilities:
one I knew about before; your colours on map_ground_types or map_climates don't tie up with the recognised RGB values - watch out for using brush / non opaque setting and getting merged colours on your tga's.

one Dol Guldur recently pointed out to me; for some other reason it is not finding your terrain/aerial_map/ground_types/texture, which could be:
name doesn't tie up with descr_aerial_map_ground_types.txt / doesn't exist / you're trying to read it from mod folder and haven't got right amount of ../../ in text file path to make it read from there etc...