Issues with my baked lightmap

Hello dear lighting experts,

I came across an annoying issue with my baked lightmaps.
I have two rooms as one Blender object and want to create a smooth, even light in both of them.

Now, while the light is baked flawlessly in the big room, the smaller section stays grey, dark and/or flawed.
So I do not understand why the lightmap renders a correct, bright light in the big room but not in the small room with the very same emissive ceiling lamp materials.
The wall material itself also is emissive to achieve an all-white, sterile look.

This occurs both with the Progressive and the Enlighten lightmapper. The Enlighten mapper, at that, throws ugly dark low resolution artifacts in the smaller area.
The smaller room shall look just as bright as the big one.

The first screenshot was made with Progressive CPU.
The second screenshot was made with Enlighten. The Game view result in the big room is what I desire the lightmap to look like.

Thank you in advance for your help.


Progressive CPU


Enlighten (Desired look in big room)

 

I tried to improve on those results in the past days but I cannot get it better even with split geometry areas and increased Lightmap setttings in the Model. Need help.