I wish it to be so simple and straightforward.
Please see my correspondence with Serbian embassy in Havana below:
Dear,
Aside of valid Agreement on free visa system between Montenegro and Cuba,
Cuban citizens should have the invitation letter made by an citizen from
Montenegro, who will guarantee its staying.
Reservation for hotel will not guarantee of entrance to Montenegro.
It is the discreet right of any police department in the world to decide if
someone (even with visa) may or not enter its country.
Best regards,