Hello ,
Unreal Engine 4 has the ability to control the number of Indirect Light Bounces as well as the smoothness of the lighting. This seems to be related to the Indirect Lighting as you mentioned yourself. Try going into your light source and increasing the number of Indirect Light Bounces as well as the quality. After making these changes, rebuild your scene and see if this changes anything.
Here are some image references to help you locate the settings I am mentioning to change.
Directional Light Settings
World Lightmass Settings
If you have tried a number of these options and exhausted your efforts, and still cannot get your black spots to go away. Let me know and we can try some other options.
Thanks,