Windows 10 Home 64 bit Build 14393.
GPU is an AMD R9 390 running on an older Crimson driver (16.7.1) because of stability reasons.
Broken: 2.78.5 4e22e24 and 2.79 248bba81e7a
Worked: 2.78c e92f235283
Short description of error
I can't change the Cycles Compute Device to OpenCL and use my GPU for rendering. Earlier versions work fine. I want to use these versions because of the Principled BSDF shader but it's not really of any use if I'm stuck with CPU render.
Let me know if you need anymore info.