Thanks for the answer!
Unfortunately, it is not the water, I used the water plugin of Unreal for smaller rivers (not in the screenshot) and a big default plane with a water material from this pack on Fab (super cool and free too btw! 
)
Without any water related actors in level:
I also tried to create a new landscape on an empty project with even no material on it and the problem persists for some reason.
Empty project landscape with no material:
Maybe some Lumen settings i have to enable?
Edit: The problem is still there even with a plain landscape with no material!