Hm that’s interesting. It triggered earlier while I was playing with materials. The editor struggled to redo the shaders when I kept updating materials and then lighting broke. I haven’t built lighting all day so it could just be an issue with Unreal struggling to handle shaders when there is a lot of complex geometry, big lightmaps, lots of reflections etc. Do you have a lot of materials in your scenes? I have hundreds.
I also found a much better way to fix the issue: create a new project and migrate your map into the new project. Everything required for the map should come across so you just have to fix up the project settings (including player controllers etc.). I’ve successfully built lighting in a previously broken map this way. This makes me think that there should be a solution somewhere in the project files, like deleting a corrupt file.