On any level with more than one or two meshes and/or some custom materials, Lightmass reports:

Couldn't gather the requested number of indirect photon paths! 0 gathered

and fails to bake any indirect lighting. I've tried numerous times to reset every possible lightmass settings, but I can't find any reason behind this.

Furthermore, it appears to always work with built-in engine materials, and sometimes it works with any built-in engine material and one custom material, but will not work with one or two custom materials reliably.

To test this, I even created a stupidly simple level with two planes 300 units apart, facing each other, representing a floor and a ceiling. I placed a bright spotlight facing the floor. Additionally, I created a blank material with its base color set to white.

I should expect to see the spotlight reflected off the pure-white ground and onto the ceiling, back again, and so forth.

However, I don't see that. I see nothing, because it gathers zero indirect photon paths, for some reason. If I revert all meshes back to their default WorldGridMaterial or BasicShapeMaterial , it magically works again.

I've built lighting so many times over the past two days trying to figure this out before a deadline that I can't remember if I had any successful lighting builds after updating to 4.20, but for the same levels this issue never occurred in 4.19.

Further notes:

CPU: AMD Threadripper (in case this is an issue with Lightmass using Intel-specific instructions)

UE4 is set up to use forward rendering and VR.

These failures behave the same with or without using normals for static lighting option.

Fails with or without material layers enabled. I had original thought that was the issue.

All Lightmass options are default

Fails with or without planar reflections

Fails with or without high-quality reflections

Fails with or without two-sided lighting/shadows

I cannot think of a reason for this. This is absurd.