4.15 can't build lightning

I’ve installed UE 4.15 today and I’m experiencing a critical issue: I can’t build the light anymore, cause the Editor would either crash or freeze for a couple of minutes before returning me an error log saying that the lightning could not be built.

I’m not working on a big map with lots of meshes or anything, I’m using the almost empty FPS demo map! I simply added one tiny Cube mesh and I can’t build the project no matter what I do.
I even got a Windows alert message saying that Unreal Lightmass is draining my PC’s memory.

For a moment I thought something was wrong with my hardware, but I tried to replicate the same conditions on 4.14.3 (with the 4.15 project still open and running) and it worked smoothly.
Any suggestion?