It seems that the lightmap for any project in which lighting was built before 5.5 can not be loaded in 5.5, and lighting needs to be rebuilt in the 5.5 editor.
Recreation:
- Create an empty project in 5.4.4
- In a fully empty level add an object or two and spotlight
- Build lightmaps
- Open the project in 5.5.1
- Lightmaps are not loaded, UE complains lightmaps are not built.
Bug has been reported as Case # 15933104
Urgently seeking any possible workaround for this, thank you.