Building Lighting Never Ends

Ever since I upgraded to 4.9, I can no longer build lighting for any level. Even if I create a blank new map, it hangs forever at Building Lighting 50%. There is 0 cpu activity going on in task manager.

This is all that shows up in the log:
LoadErrors: New page: New Map
MapCheck: New page: Map Check
Cmd: MAP REBUILD ALLVISIBLE
LogEditorServer: Rebuildmap Clear paths rebuilt
MapCheck:Warning: Warning WorldSettings Hierarchical LOD System is disabled or no HLOD level settings available, unable to build LOD actors.
LogStats: STAT_HLOD_BuildClusters - 9.051 ms
LightingResults: New page: Lighting Build - Sep 16, 2015, 1:18:40 PM
LogStaticLightingSystem:Warning: No importance volume found, so the scene bounding box was used. You can optimize your scene’s quality and lighting build times by adding importance volumes.
LogStaticLightingSystem: Running Lightmass w/ ImmediateImport mode ENABLED
LogStaticLightingSystem: Running Lightmass w/ ImmediateProcess mode ENABLED
LogStaticLightingSystem: Running Lightmass w/ Sorting mode ENABLED
LogStaticLightingSystem: Running Lightmass w/ Mapping paddings ENABLED
LogStaticLightingSystem: Running Lightmass w/ Mapping debug paddings DISABLED
LogEditorBuildUtils: Build time 0:00:03

This looks to be a similar issue that has already been reported with UE-20782.

Unfortunately, this isn’t affecting everyone, but it is affecting some and there are multiple ways to cause it to hang. When I reported this I have 3-4 different ways to cause light builds to freeze at 50, 75, and 80% completion.

One thing you could try that worked for me in a simple scene is to add a default sized BSP cube to your scene or the SM_Template_Map_Floor mesh that is from the Default Level. I cannot guarantee this will work, but it has helped when I tested this originally. If we end up making a 4.9.2 (not guaranteed) and this has been resolved by that time we will push to get this fix in.

Sorry for any inconvenience it’s causing.

Tim