I was having the exact same problem as well, MAKE SURE that the path to where you specified you had/have your punbb forum on in your site is exactly the same as what you specified it to be on the installation. Like durring the installation, if you specified it to be www.yourdomain.com/forum, that's EXACTLY where the forum has to be, and the name has to be exact to (ie www.yourdaimhere.com/forumname). Hope this helps,
Brian