They Ryan, thanks a lot for the answer! 
I have fixed the issues you mentioned, I removed the blending of the displacement map and I also subtracted it by 0.5, the material now looks like this:
http://puu.sh/jLVeU/aafc56ad06.png
The problem with this is, it is now also displacing the landscape in the background, so there no tesselation gets applied, but it’s still displacing the polygons:
http://puu.sh/jLUVH/2307c193b4.png
And this also looks really “cracky”:
http://puu.sh/jLV1F/1fa6e7a4ce.png
The screenshots above are taken with “Displacement Multiplier” set to 1000. If I set it to something lower, like 100, it looks less “cracky”:
http://puu.sh/jLVa1/9ce59cf823.png
From near the ground it looks like this (multiplier set to 100):
http://puu.sh/jLVqI/14c3432167.png
http://puu.sh/jLW18/e045196deb.png
It feels like the displacement gets applied at the totally wrong location. It should make those stones “come out”, but it seems to just displace the whole landscape in “waves”.
Where can I set the resolution of the landscape? At the moment it’s set to 2 polygons per square meter. I can’t find a setting for changing this, neither in the landscape actor nor in the landscape generation settings.
I know how to build UE4 from VS, do you think the problem is just the tesselation resolution? At the wireframe screenshot above you see that the resolution is not really “high”, but i should at least be enough to make one vertex pop out where the stones are. If it would work correctly, it would probably look quite “spikey” because of the low resolution. But there has to be another problem at the moment since it does not even look “spikey”…