Error #0089: you cannot open a private chat with user UIN XXX because they are not in your list of general contacts (you have a restriction on the server)
You are not allowed to open a private dialogue with a user who is not in the common contact list.
Three variants:
1.In Admin Panel, in the section "Rights control", "Permission groups", "Privates" disable the rule "Forbid private conversations with everybody, besides users, who are in the common contact list".
2.Move a user to a group with no such limitation.
3.Add a specified user to your personal contact list.