Hi,
that is a bug which I've fixed in one of the v3.x updates.
Since the symmetry object only processes polygon objects and no folders it stumbled of the folder. So don't drag and drop folders in the symmetry object if you are using Cheetah3D 2.x.
If you can't open the file anymore just email it to me (ziped). I will open it v3.x and resave it with the folder out of the symmetry object. So that you can open it again.
Sorry for the problem.
Bye,
Martin