Thank You for the help and suggestions.
I managed to fix this problem on ubuntu/linux with IPV6 disable actions.
It looks like that was the main cause in my case. But I'm not entirely sure if the problem is ubuntu last version (20.10) or (RPi-4...) latest firmware updates. My problems started
after latest updates. Everything worked well on ubuntu 20.04 for last 6 moths..
Anyway, hope this can help to someone facing same problems as me...