Is it about the lighting gradients?
I notice that in the screenshot, the lightmap tries to pull off a gradient on the ceiling, but in 24-bit RGB, it's a little bit limited and you get unnatural colour "bands", noise in the colour, because there are only 256 levels of lightness available.
Try using some less-clean textures, they can help get rid of the colour bands.
This occurs when a lightmap tries to pull off a smooth gradient, but the pixel format while rendering doesn't actually provide enough colours to smoothen them well enough.