Steps to reproduce:
1. Open "case 805857 repro" project
2. Open "example" scene
3. Play in editor
-- Notice brown illumination artifacts on the darker side of 3D object (gif attached)
4. Change Ambient source intensity in Lighting tab back to 1
-- Notice that illumination artifacts are gone

Results: Changing ambient source intensity value higher than 1 produces illumination artifacts when procedural skybox is used.

Note: You change intensity to any value and it will produce different artifact (gif attached). In this case was used "Star Nest Skybox" asset which is compatible with Unity 5.3.5 or higher.

Reproduced with: 5.3.5p3, 5.4.0b21