Ullman: You have enabled the 'Reject_Unknown_Ips' option in the .ini file and the IP/hostname of the user trying to connect doesn't match ANY hostmask across ALL the users so the server won't talk to him.
You can probably verify this using the 'site findip <IP>' and then 'site findip <hostname>' option which won't return any matching user hostmasks.
Of course the solution is simple, just add a new hostmask to the user
