Hello hiriperi!
I will try to explain it.
This is about baking the Displacement.
If you enable Bake from Multires, Blender calculates how much the ‘Render’ is displaced from the ‘Preview’. It will color each pixel in your map black where it is displaced ‘down’ and white where it is displaced ‘up’ , middle gray where it is not displaced at all. (And everything in between of course…)
Now if you have your ‘Preview’ resolution set to the same as your ‘Render’ resolution, there is no difference between the two and therefor no displacement, leading to a completely grey map.
Hope this helps.
