Viewport turn black in iray at certain camera angle

I have encountered this issue recently. When I rotate the camera around the characters, the viewport turn black at certain angles when it is in Iray, in both perspective view and camera. Happens too when I move the camera nearer to the characters. I have checked that there are no obstruction (wall or column) around the camera. Scene is well lit with multiple lights. No such issue when not in Iray mode. First time I encounter this for this particular scene.

Anyone seen this before? Any possible solution? Thanks!