I am trying to render an interior image but I have problems with the white walls. As shown in the image attached there are "stains" caused I think by the DENOISER. (I guess so because of a tutorial I have seen on youtube). The render is running on my Macbook pro mid 2014, and the rendering settings are the ones of the screenshot attached. I don't need any high resolution image but just uniform surfaces for a quick preview. As lighting: The scene has an area light in front of the window hole as "portal" and an hdr image attached. The walls have 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

