Hello wukakuki,
I have been able to track this down to a known bug where the lightmass is running but the build hangs at certain percentages. We are currently trying to track this down as this seems to be one of the larger issues with 4.9.1.
The ticket number is UE-20782.
I will keep you posted on this bugs progress. I hope this issue is resolved very soon.
I would recommend trying to build this in 4.8 in the meantime.
Thanks,