I had a project in 4.76, and after I converted it to 4.9 this error started popping up. It also happened when moving from 4.7 to 4.8, which is why I didn’t make that switch first. It doesn’t happen in all of my scenes, just the majority of them, especially the larger ones.
Essentially, I load into a scene and everything seems fine until I play the level, then the unbuilt lighting message pops up. If I stop playing, it goes away. If I rebuild lighting, everything works right, no message either during play or otherwise, until I save and reload the level, then it acts as before, showing the error message if I hit play.
I found several similar questions, but none of the solutions helped here. No terrain/foliage is in use, and while my levels use construction scripts, I tested removing all blueprint functions from the levels and it still occurs. I deleted basically every temp file associated with Unreal and no change. My UE4 installation is verified and proper. Migrating the scenes into a new project migrates the error with it. Using DumpUnbuiltLightingInteractions just gives a list of assets flagged, removing them doesn’t actually fix the issue, and is also really difficult/painful in a large scene.
I did a test, and it appears that some of the models in my scenes are broken in some invisible way. Rebuilding the affected scenes with exactly the same layout and with exactly the same assets does appear to fix the issue, but I have like 30 scenes all built with modular assets (i.e. each scene is made up of like half a thousand parts). This would basically be building everything again.
I managed to build a stripped down test scene where I can show the error with just a basic scene containing nothing but a few static meshes and a blueprint that I use as a pseudo-light source, that is just a few static point lights and a mesh. I would really appreciate if any solution, work around, or at least some way to identify the affected actors so I can replace just them, rather than the whole level.
Here’s a link to the example project if needed: Dropbox
It is just a project containing two example scenes, the one with the bug that was originally upgraded from 4.7 then migrated to this project, and a copy of that level I rebuilt from scratch that doesn’t seem to have the bug.