Hi Pilou - I believe that you may have accidentally selected the bottom rectangle for the one that gave you the unexpected result.

For example, selecting the box and the curve:



However, if you select the box, curve, and the bottom rectangle:



So the key difference there is the bottom rectangle being selected or not.

When that bottom rectangle is selected, it will become automatically extruded upwards, that is the source of the extra vertical material that you see there.

I think to get the result that you wanted, make sure not to select that bottom rectangle.

- Michael