Mysterious story continues...
After 2 months I take the MB in my hands again (meanwhile it was just lying on the floor) and guess what - the problem with KB and mouse is back, exactly same behavior as when I started testing it. So first I desoldered that suspicious 470p cap. network but no effect, problem remains. Then I tried to pull out and push back the CPU in slot1 and pushing on BGA chipset and various bending the MB but nothing helps. So this proves it must be some broken solder joint or via, not a degraded component in KB circuit. As a last resort I could try reflow the BGAs if something changes but I don't have much high success rate with it, may it ends with total damage of the MB...

I tried to bake the chipset but there was developed a short under BGA, so giving up, back to trash, problem solved...

Hurts to read this 🙁