Hi Pilou, yes offset can not currently take all those curves at the same time.

If you follow the steps here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2088.5
You'll see that I select only one loop at a time instead of everything all at once.

You can select more than one curve to offset, but not when they touch like that, it tries to join those touching sections together into one loop and you've got multiple branches in there instead of one single loop result.

- Michael