Weird arctifact with AMD GPU rendering on Windows (it worked well, in past) [UPDATE]

I’ve an AMD R9 280x, and I was used to use GPU rendering on blender. It worked well, from 2.7x.
Then, for some weeks, I used progressive refine, with no problems.
Since I noticed that progressive refine was veeeery slow, I disabled it back.

At that moment I noticed a problem with the usual tiled rendering on GPU. With CPU rendering I have no problems.

EDIT: on 3d view, GPU rendering works well. It is only the final rendering that does a mess…

CPU render example:

GPU render example: (the gray resulting color and tonality seems to depend by the scene medium color)

What can I do?
I’ve tried to uninstall AMD drivers and reinstall the latest stable driver version. I also reverted blender settings to default ones, to change again to OpenCL to enable the GPU rendering. Still same problem.