Hi Pilou, the line that doesn't work is going from the lower bottom corner to an upper corner, so it is diagonally slanted in 3D, it isn't aligned with the construction plane.
In Booleans when curves interact with solids, the curves must have a well defined projection direction perpendicular to the curve, so the curves must be planar like a circle, or if they are a line they must be parallel to one of the world xy/yz/xz planes.
If you draw your line along the bottom corners of the box so that it lined up on the xy plane, then you can do the merge as you want.
 Michael
