'
Forum URL' must be correctly set.
But you should also check out this option: '
Cookie Domain'.
To allow users to stay logged into the forum if they visit via either,
http://www.FORUM or
http://FORUM you should select suggested setting which looks like '.yourdomain.com', then save settings, then log-out/log-in. This may solve your loading problem.
Be careful when setting this as entering an invalid setting can leave you unable to login to your forum.