Hi Phiro, the weird piece that looks like a curve is a surface but with a messed up trimming boundary on it which prevents it from getting a proper shaded display like surfaces normally have. If you select it and use Edit > Show pts you can see the surface control points and if you select its 2 edges and use Delete to untrim it that will recover the surface.

- Michael

Thx for the explanations!
A little more clear now! :)

And no need to make a plane a simple line is sufficient ;)