nope I used the last Unwrap 3.16 and the last FBX plug in version

I got the answer from Brad
FBX use Euler angles only
If some rotational keys of your model are close to +- 180 °, you can expect problems

So it's always a gamble when using FBX? Odd

Thanks for telling

I reset to zero all the rotational keys ( MilkShape provides a zero joint tool )
so I managed to have some animations working also in FBX
I have of course to throw away hundreds animation sequences which I created in years time
Not a nice situation

Indeed. I feel with you