Shadow quality can be very poor when you use light fog.

This may be because your Cone Angle is much larger than the region of interest, and use dmap auto focus is turned on. When use dmap auto focus is on, the renderer computes the smallest angle to obtain the highest precision on the shadow of interesting objects. But when light fog is turned on, the angle must be as large as the light Cone Angle.

Turn off that attribute, then adjust the Focus in Depth Map Shadow Attributes accordingly to fit the scene you are interested in.