I cannot add very much to this error message: Your mesh in Blender is not ok... something is double... :-/
Indeed, It was a problem in Blender and not in LOTUS C.T.
For future people: I just follow this on this thread (https://blenderartists.org/t/remove-double-faces/582568)Quote
Press A twice to select everything then space and type in remove doubles.
After that if you need it again just press space and click on it.