It seems there is (still) a Bug with the null material in 0.8 stable.
I use a null material blended diffuse material (no bump) Hybrid + BiDir + MTL!

(540 mutations per pixel)
for everybody who can't see this, I raised the contrast in PS.

Here is the Scene File:
***EDIT***
I let the rendering stay over night, maybe this artifact is gone then...
I tried the same scene used diffuse transmiiter at 1000 Muts per pixel,
but no artifacts are visible...





