Strange square lightmap artifacts

Hi, I’m very new to unreal.
I made an interior scene that I’m trying to light properly, but I always get these strange square shaped artifacts. I noticed these artifacts are most often found in parts of the meshes that are invisible (completely occluded by other polygons), but they sometimes expand into parts that are visible. Every time I change the mesh and it fixes it in some area, it appears in an other area.

I don’t know how for sure, but it seems like unreal tries to determine what parts of a mesh are visible and which parts arent, and then uses the invisible parts as buffer/bleeding area, but in my case, it mistakes visible areas for invisible areas.

I looked at a lot of lighting problems and tips, but I can’t seem to find anyone else with this problem.
Does anyone know what causes this and how to fix it?