When I set light source intensity about 3.0lux, and use atmospheric fog sun disk, close auto exposure, set EV100 to 0, screen starts flicker, the following pic show the problem.
normal:
when i move mouse:

When I set light source intensity about 3.0lux, and use atmospheric fog sun disk, close auto exposure, set EV100 to 0, screen starts flicker, the following pic show the problem.
normal:
I had same problem yesterday.
Try to update graphic drivers, restart system and open UE again (it worked for me).