Swarm can't finish building light even for simplest level

Here is my level.

Used cube and sphere from default primitives. 1 point light and Lightmass importance volume.
When I’m trying to build light it goes instantly to 80% and getting stuck there for 1 hour+. I think it is more than enough to calculate light for this setup even on weak PCs.

I had few successful builds using modified firstperson example, though it was inconsistent. Something like:
build light as is - fail
build with 1 more cube - fail
build with 1 more cube - fail
build with 1 cube removed - success
build with 1 more cube - fail
build with both cubes removed - success

What am I doing wrong?

Version: 4.9.1-2689999+++depot+UE4-Releases+4.9

Hello dimwalker,

This appears to be an issue where the lightmass is continuing to run while the build is stuck.
This is an issue that our developers are aware of and are currently working to fix.

The ticket for this issue is UE-20782.

Thank you very much for reporting this issue. I will keep you up to date on it’s progress.