Hi Michael,
Fist of all, thanks for your software, it's a pleasure to use.

I think Pilou was right, my mesh was not closed and this caused the mess. Here is a picture of what i see in modo just after import :



I have to export with unjoined edges, otherwise every n-gons render with creases. But as you can see, with all the bevels this makes a fair amount of surfaces and makes it nearly impossible to manipulate or, in this case, repair.

Anyway, i probably make things the wrong way, either in the construction method or in the export process, but i spent 3 hours learning the soft and was able to do the job, so once again : wonderful soft. (i would have spent days doing this in modo...)

Of course, if you still have an advice for my case, i'll take.

Gep.