jsmack
You should not get black interiors, MaterialX in Karma enables fake caustics by default. You only need to enable caustics when you want cool patterns, not just to let light into an interior. Even with how fast XPU is, if you use true caustics you'll need 1000-1000000x as many rays to resolve it, as well as having to increase the color limit for indirect light to be as bright as your light sources.
Understood, heard on this, and thank you (both) for it; I will revisit this bit of my setup to optimize this part of it (I've been working on this for over a year or so, so it's got lots of warts as both Karma and my own understanding of USD have grown).
jsmack
Disabling caustics is all it took for it to work for me. I think there is bug there somewhere.
So do I understand you correctly that you can open the scene file in the Dropbox folder above, and it renders fine for you as-is? In that file, I have already disabled caustics on the renderSettings node, and it consistently fails for me...which indeed seems like a bug.