www.kraytracing.com

Could you help me solve these issues?
My render set-up is:
Photons preset - PM
1 spot light as Sun, 1 lumi poly in window as Sky

The artifacts on the reflections in the lower left is caused by the blurry reflection accuracy value. Basically, if the amount of reflection blur is above the value set in Kray, it will only use the precomputed irradiance map for reflections, instead of firing new final gather samples. So what you're seeing in the reflections is the irradiance map. Either improve the quality of the map, or increase the accuracy level to something above the amount of reflection blur on the metal.

Thank you Captain. It was exactly as you wrote. I changed also FG set-up (now higher numbers in spatial tol. 0,15 and B/D 0%) and problem with the corner disappeared. There are still some white splotches on the window frame

Could you turn on "show fg samples" and render that out? It's best if you then also set prerendering to 0 % and turn off anti-aliasing to save time.

Do you have an area light or lumi polygon outside the window? If yes then move it a bit further away and the noise should be gone.

Jure: I have a lumi polygon outside the window. I tried to move it but without any success.

I changed material for this curtain and it is better. No Transparency - only Translucency.

Much nicer! Now set the "Corner" distance in the FG tab to 20 % or so, and the paths to 5. That will improve contact shadows and remove light leaks.