• Volumetric fog can cause seams because volumetric effects are aligned to the camera frustum (the camera’s view). Since we are stitching 6 camera’s together, frustum based effects will cause seams due to a mismatch in what each camera is seeing.
• Directional light and sky light settings can also cause seams with Volumetric Fog effects. 

For the best results:

1. Checking if you have an Exponential Height Fog Actor in your scene by searching for it in the Outliner:
2. If you don’t have one, add fog to your scene by going to Place Actors > Exponential Height Fog:
3. In Details of the Fog, set a value for Fog Density. Try a density of 4:
4. Fix the color of your Fog by going to your Sky Atmosphere Actor and changing the Rayleigh Scattering colour to something more grey or white:
5. Make your fog more interesting by adding Volumetric Fog in the Exponential Height Fog Details Panel:
6. This will cause some seams in the shadowed areas where objects occlude the Light Sources in the Scene. This can be more or less visible depending on the scenes lighting and layout.
7. Some seams here can be fixed by going to the Directional Light and unchecking Light Shaft Occlusion:
8. In more complex scenes where the actors cover the sun more or have more complex shadows you may have scenes caused by Ambient Occlusion.
9. To fix these, add a Post Process Volume as described here, then go to Ray Traced Ambient Occlusion and set Enabled to True and Intensity to 1:
10. You will lose some lighting detail when enabling Raytraced Ambient Occlusion. Some of the darker areas can be restored by upping the Ray Tracing Ambient Occlusion  Radius. This will have a smudgy effect on the shadows however so should be used sparingly:
11. If you don’t want to enable Raytraced Shadows, you can also try Disabling the Cast Volumetric Shadow setting in the Skylight., this will not totally fix the seams, but will reduce the contrast in the volumetric shadows:
12. Final tip: Sometimes artifacts will occur only when the camera is moving, and the seams will clean themself up after a period of time. In this case you can use Anti-Aliasing to remove the seams as described here.