Lighting bug - overexposed/posterized in build

(My apologies if this bug is known or fixed or something - I searched but could find no information nor a patch.)
I have encountered a pretty mean bug with the light system.
It occurred in a VR-project targeting the Vive that also uses additive Scenes. But the bug is still there when I throw out SteamVR and all of the other scenes and disable VR altogether. The project and the scene uses Realtime GI.
In the project everything looks fine. In the build everything looks fine, as long as I execute it on the same windows user account that was used to create the Unity project. However when I try to run it on a different machine or even on a different account on the same machine the light is broken. Looks overexposed to me and kind of "posterized" for lack of a better word.
This is the second time I come across this bug, both times were in Unity versions <5.6.
I tried isolating and updating the affected scene to 5.6 and rebaking the light, but it did that did not help at all, it was still broken.
Last time I encountered the bug I tried throwing out basically everything but a single mesh and still couldn't fix it. Deleting the library folder, rebaking the light multiple times, tried everything. The only thing that helped was rebuilding the whole project from scratch. It wasn't too bad then because it was a small project but this time around the project is much bigger. The problem came suddenly when I was working on a different scene and due to the strange behaviour with builds looking good locally, I didn't immediately notice it. It seems to occur pretty randomly but once there seems impossible to get rid of.
I'm filing a bug report that also will link back to this thread.
I'll also add two little screenshots in a second. Here they are:

Any help is super-appreciated! I know it's nearly Christmas, so I'm patient, but will be visiting this thread from time to time over the holidays in case somebody has the time to answer. Thank you!