Is there any reason why a user might not be able to post in a forum they have permissions to post in? They keep receiving the message 'You do not have the right to post an item in this forum' but they do have permission. Might this be security related?
Yes. Make sure (delete your own cookie, then log back in as Admin) the user in question is in the correct group to post. If all looks good on your end, then have the user log out and delete his cookie for the site and try logging back in again.