Hi Lina,
It was quite a while ago, but I think there are a few solutions.
The black line can be the lower hemisphere of your skysphere. So either raise (or lower) your skysphere until the black line disappears OR make it so the lower hemisphere of your skysphere is the same colour as your upper hemisphere.
I cant find the settings right now, after a quick look you can find a ‘horizon falloff’ setting inside the skyspheres details panel.
Messing around with that may also clear/reduce the black horizon line.
Let me know how it goes.