Win10 64bit, 2x GTX970, i7 5820K
Short description of error
The portal planes divide the scene into smaller parts and some of the parts are rendered darker than the other.
Exact steps for others to reproduce the error
Open the blend file and hit F12 (Render)
The render has visible "lines" going from the intersection of portals - even over the monkey.
Disable the second layer with portals and render it again - now the artifact are gone.