Mesh conversion problem form 2.1 to 2.6

[acme3D]

I recently upgraded my 2.1 built project to the new born 2.6; some characters in the game have now missing polygons and are badly damaged in some areas.
If I newly import the FBX file then things go bakc ok; in the picture you can see on the right the char converted by 2.6 and on the left the same file duplicated from the finder and newly imported…
Somebody already had this problem and found a solution ?

[antenna-tree]

What happens if you manually choose to re-import a mesh from the Project View (not duplicating it first)?

[acme3D]

Hi, it happens even with manual reimport… as you can see from the attached image…