Thank you Falko.
I came very close to the solution but I have not reached yet.
I am at the last step of this guide:
I can connect with multiple VNC session but after entering password I see black screen (as expected).
Now I should edit the file $HOME/.vnc/xstartup as described but this not solve.
My xstartup file (please see screenshot) is different from that in the example.
doesn't solve; I still see a black screen in VNC.
Any help would be appreciated.