Just as Geza Kadas, GPU Lightmass started crashing for me today. Haven’t done anything special that I know and I have done several light bakes today without a problem, and now my light bake was just interrupted by this nice little message:
If I click OK, I get the option to abort (which aborts the bake), retry (which immediately crashes) or Ignore, which just goes back to getting the launch failure, and around and around 
Except for this strangeness, I’m quite impressed with what you’ve achieved; I’ve rebaked several old projects, and it is 5-10x faster AND gives higher quality than I’ve had before =D
EDIT: Apparently there is something in one of my levels that causes the crash, cause when I tried baking ligt in other levels in the same project, there was no problem, Didn’t even restart my computer or UE. There was really nothing special at all in the scene as far as I can see, just a few meshes, a few lights, a skylight and a postprocess volume, more or less. Any idea what could have caused the problem?