Cannot build lighting (Mac)

I am making a 3d sidescroller game demo and lighting will not build. I left it to cook overnight and it was still at 0% this morning. Some kind of menu pops up with a progress bar, which flashes by too quickly to read, then the “Building Lighting… 0%” shows and the icon animates but it’s not doing anything productive.
Activity monitor shows “UnrealLightmass” taking up 0.5% CPU resources. Beyond that I see nothing else going on in the background.
Then it just sits and thinks about nothing for however long you wish to let it sit and ponder (24+ hours and still 0%). It doesn’t crash, but it doesn’t go anywhere either.
The only lights in the scene are a skylight and an omni light, the same default stuff that came in the template, unmodified.

I tried this same experiment with a fresh template level (3d side scroller), brand new, right after reboot, and it does the same thing. So I don’t think it’s my level specifically (all I did to mine was move some of the default sidescroller blocks around to make the scene wider).

Machine is an iMac, 3.5ghz core i7, 16gb RAM, running Yosemite, unreal engine Version: 4.8.1-2591939+++depot+UE4-Releases+4.8

I read the article Why can I not build light and swarm fails, and why doesn't light bounce in my room properly? - Rendering - Epic Developer Community Forums but it seems irrelevant.

Installed UE4 Windows in virtual desktop, loaded the same project in that, and the lighting built in less than 5 seconds. Why it doesn’t work on Mac, seems that the swarm launch thing does not start. Tried reinstall on mac, swarm is still broken by default.

I’m having an extremely similar problem on my mid 2010 2.93Ghz Intel Core i7 iMac. Leaving lighting to build overnight and nothing happens, or the computer locks up.