Could it be that the user he ignored was at one time a regular user, and now that user is a Moderator/Admin -- which are exempt from being ignored?
Also, This not an answer to why it happened, but a possible solution to fix the problem (or find out more about the problem by producing a "no access" or "database error"...etc message) you could go the direct route and send him a "Toggle Ignore" link with that ignored user's number:
http://[WBSITE-URL]/forums/ubbthreads.php/ubb/toggleignore/User/[USER-NUMBER]
