You know, some strange thing happened.
Trying to solve this problem I started to change positions of those sticks again, and at some point I've got 40 GB of ram as usable. So, I just continued to change them on slots, install only part of them and so on over and over again...
And finally - SURPRISE! - I've got 64 GB of usable RAM! The most strange thing here is that I've got it installed 4 old normal size DIMMs in combination with 4 compact size DIMMs which I purchased first, as I actually had from the beginning of this story...
I don't see any logic in this, so I think computer just needed more love, or better to say wanted me to make love (let's name it so) a bit with it. :)
However, now I have whole 64 GB of RAM, and still can return 4 new compact sticks back to the seller, what is actually good!
Thank you for support!