I had the “Cast Modulated Shadow” box become greyed out spontaneously as well. I had checked it a short while before, then unchecked it, and did some testing the dynamic lighting. When I went to check the “Cast Modulated Shadows” again, it was grayed out.
I got around this by creating another directional light. With the new light I had just created, I could check the “Cast Modulated Shadows” box as it was not greyed out.