Strange shadows between attached objects

I’ve been struggling with this strange shadow artifact when putting models (from blender) side to side. The two pavement objects are plain six-faced cuboids with no fancy geometry and they are correctly UV mapped. The light in the scene comes from two planes on the ceiling with an emitting material.

I’ve tried countless times to attach the objects using the grid or the vertex snap while rebuilding lighting everytime in highest quality and this is by far the best result I had.

Note that the vertical artifact on the walls is not actually an artifact, I’ve modeled the walls to have that gap. I’m asking help for the artifacts in the pavement objects which, as I said, are simple cuboids.

Hi!

Well if you see “random” shaped blurred out shadows you can be sure that your lightmap resolution is not high enough… (or maybe just not grid/pixel snapped)
In Unreal with static lighting all assets are calculated separate so even snapped meshes will have shading differences with default lightmass settings! You can avoid it with ILLS: 0.1 and ILS: 0.6!

Sorry I also forgot that you’ll need to have (extra) portals to achieve nice lighting from emissive materials…