I have the same problem in UE 4.15 … I was using Cast Modulated Shadows on the stationary directional light in my mobile project. I turned on Cascaded Shadow Maps by setting Dynamic Shadow Distance StationaryLight to 2000 with 1 cascade and no inset for moveable objects.
Now I can not turn it off. If I set the Distance back to zero, The Cascaded Shadow Map Options grey out. But ‘Cast Modulated Shadows’ is also greyed out and I can not interact with it.