Lighting Artifacts on the Ceiling

I changed the Mode property of the directional light source from ‘Realtime’ to Baked and the following artifacts appeared on the ceiling and walls.

Why? How can I fix it?

I’ve done the followings without success:

  1. Changed Light source type (spot,
    directional, point) → no success.

  2. Changed environment reflection > Bounces from 1 to 4 → no success.
    Increased resolution of environment reflection → no

  3. Changed all various settings in light mapping settings (11 parameters) with Enlighten → no

Those artifacts was due to lightmap UVs. There were missing or corrupted. In the import setting of the ceiling and walls FBX files, I ticked “Generate Lightmap UVs” option and voila! all artifacts went away.