Why would not having mesh thickness cause a black lighting? It would cause leaking, but I don’t know how it would cause it to render black where bounce lighting should light it up.
Is the mesh lit on the outside of it? What are the directional light settings? It’s a directional light that sometimes requires modifications to receive more lighting indoors, while skylight needs a boost in intensity scale to do the same (or a change of cubemap). The issue is most likely either directional light and/or settings for the mesh. Post a screenshot of all lighting settings for the directional light, and the Build settings of the ceiling mesh (not the new UE cube one you made, but the old one if it’s still in the scene), and I’ll have a look at it, try to understand by testing the settings myself perhaps.