Be sure that you have proper url's in the referrer check as well... By default it should just be your url, but keep in mind that these are DIFFERANT urls and both need to be in the referrer check:
http://yoursite.tld/forums/http://www.yoursite.tld/forums/Additionally, some firewalls and virus scanners do not send the http referral as a "Security" measure, this is a completely idiotic setting within the users software which nerfs security on your forum; you should obtain weather or not they're running a virus scanner or firewall and if so what versions; they'll need to disable this security setting.
Also, i would highly advise against ever disabling the HTTP Referrer Check.
