For some unknown reason, my AO texture plugged into the ambient of the material does not render very well. In fact, the AO is almost invisible. But plugged into the diffuse, everything is ok. The UV channel and the RGB settings in the properties are ok. So I wonder why the shader render my map very subtly ?
AO in the Diffuse texture space (Floor only)
AO in the Ambient texture space. (Floor only)