I'm having the same trouble, but I've identified the culprit in my case: If I have a Framed Text area with an freehand egg shaped path for a frame, the file won't save. If I delete the framed text, the file saves.
Here is the object that causes the problem:
Here is the message on "save":
If I copy & paste the framed text to a new .afdesign document and attempt to save it, the new document will not save. (The paste does not quite work correctly either, the frame & text are not longer the same size—the frame of the text must be rescaled. Oh, and the text color was wrong!)