An issue that looms large in my mind is the choice of a hechsher name that sounds so much like another, very trusted hechsher. Call up a restaurant and ask who supervises them. If the answer is "Cup K" you might well think they said kof-k.
Also, the Cup-K is NOT included in any O list of accepted hechsherim that I have seen, such as:
http://www.ozny.org/kosher
http://www.crcweb.org/agency_list.php
Organizations that serve their communities by compiling lists of restaurants with acceptable hechchershechshers try to create complete lists. It might be that one organization might miss an acceptable restaurant or hechsher, but it is highly unlikely that a pattern of omission arose by chance. In short, that pattern is more than an "unsubstantiated rumor".