Issues with smoothing landscape


I have issues with smoothing my landscape in parts where the angle is 90 degrees.
AFTER I rebuilt the light, the lightning casts fragments all over these sections.
(It’s not the mesh itself, it’s the lightning that has major problems with these smoothed areas. Fragments everywhere.)

It is really important for me to know how to fix this. Can’t we smooth these hard edges in the engine itself? Should we use a third-party-program like Mudbox? Is there any way to smooth out the landscape in one step?
Unfortunately I can’t post the landscape (NDA-wise).

I would be glad to hear an answer ASAP. Already spent too much time on this smooth-tool.