I've had this problem on a few visuals produced recently. Mainly where there isn't enough light getting to the ceiling or dark areas of the room.
I usually set the Irradiance Map to default 'High' and 'Best' and the Light Cache to around 1200-2000.
Adaptive DMC is on 1 and 7 with a Clr Thresh of 0.001... What's the obvious setting that is going to clear this mess up?