Not sure what happend - GPU rendering worked fine before - but now I cannot get blender to GPU render, though rendered preview works(!), so it shouldn't be driver. I have an AMD card, cannot remember driver updates recently. Updated to latest after the issue happend and it didn't help.
Problem is not related to file - same thing in the new projects now.
Also it depends on the number of samples, below are the render results for 1, 2 and 3 samples respectively. On 50 samples it's like almost completely black.
Did anybody had similar issue? Is there a known solution?