Noise canceling

Hello everybody, I am trying to render an interior but I have problems with the white walls. As shown in the image there are "stains" caused I think by the DENOISER. (I guess so because of a tutorial seen on youtube). The render is running on my Macbook pro mid 2014, and the rendering settings are the ones of the screenshot attached. As lighting: The scene has an area light as "portal" and an hdr image attached. The wall has no material assigned, and even assigning materials things don't change.

Is there any way to correct this simple mistake? What am I doing wrong?

Thank you