Hi Pilou,

re:
> I had many times some failures with only boolean objects + line
> Extruding this curve make the trick at any time! :)

If you have a line that goes parallel to the x y or z axis lines, there can be multiple different directions for the automatic extrusion, and the view that you use to select the line in becomes significant.

Possibly that's what you ran into, but I can't be sure without seeing an example file.

- Michael