Smoke Domain casts grayish box First of all, I'm new to the forum so please forgive me any mistakes (also in my english).

Setup: I'm currently rendering a Smoke simulation in Cycles, for which I've also setup a ground and wall plane as a collision object, and more importantly, a shadow catcher.




The smoke domain material is the default setup created by the "Quick Smoke" effect.



The issue: When rendering the image, theres a grayish Box at the intersecting point between the ground plane and the smoke domain (which is set to adaptive, the box lines up with the edge of the adaptive domain).


(low sample demo render, also background color is set to white for clearer visibility of the gray box)

What I've tried so far:


moving the ground plane
changing the light in the scene
checking AO
Disabling "Shadow" Ray Visibility in the Cycles Settings of both the domain and the ground plane


It looks like I'm not the first one to experience this problem, but I've not discovered an answer that worked for me yet.

Any help is highly appreciated.

Cheers
This appears to be a form of z-fighting - where two faces exactly coincide. Presumably the bottom face of the Smoke Domain rests exactly on the plane. This causes Blender to have trouble in determining exactly which surface has been hit by a ray and, despite the edge of the Smoke Domain being 100% transparent, this can result in the shadow.

In order to demonstrate this effect, here's the default cube, moved 1 blender-unit up so that it's bottom face rests on the Z=0 plane with a plane added also at Z=0. The Cube has been assigned a 100% transparent shader.



And here is the render - note the square shadow of the face of the cube that should not be present :



The solution is to slightly move the Cube or the Plane so as to separate those faces - only a very slight movement is required - moving the cube only 0.00001BU up was sufficient to avoid the issue.