ok, thankyou to try to resolve this problem.
Today I found another thing about shadow catcher:
Adaptive sampling stops the rendering on the shadow catcher surfaces to its MinSamples.
Max. Samples 2000
Adaptive Sampling on
Noise threshold 2%
Min Samples 15 (very low to understand better, usually 128 is good).
The rendering begins and must render 2000 samples, after 15 samples in the areas with the noise less than 2% it stop the rendering.
All works good, but on the shandow catcher surfaces it stop the rendering at 15 samples, no matter how much is the noise.
I think the engine compute the adaptive sampling on the background (in my case full white) and not on the polygons shadow catcher, like they are completely transparent.
To get a clean render on the shadow catcher surfaces too, must increase the Min Samples in adaptive sampling (usually for the shad.catch. need minimum 256), but this is a waste of time for the surfaces that are already clean at less samples.
Maybe it is always the same bug that breaks the denoise?
Win 7 x64 | Asus P8P67-M - i72600K | 16 GB RAM | 1 x nVidia RTX 2800 | 1 x nVidia GTX 750ti