Well, I don't really know, how to describe this problem, but here it goes: I made a high poly and a low poly model of a bedsheet. Both have the same size and the normals are correct. There are no (official) errors accuring. But the problem is, that the baked Normal Map covers only a part of the UV Map (Smart UV Project by the way) -the middle to be exact. Everything is baked, but the projection is too small and sometimes it's not even the correct dimension. Now I noticed, when decreasing Ray Distance, the Normal Map grows bigger, but the problem is, when it fits the UV Map (which happens around 0.06) the rays are too short and so some parts remain black.
I'm still using 2.79 if this is any help.