Your fireboard is working fine (
http://www.tukuza.com/index.php?option=com_fireboard), the issue here is your JoomSEF SEF URL's component. I've often run into this problem and either had to change the URL, make a new menu item or in some rare instances reinstall the troubling component.
So try changing '/forum/index.php' to 'forum.html' (or whatever you want). See if this helps

If not or if you need more information about how to change the URL have a look on the JoomSEF developers website
http://www.artio.net/en/joomla-extensions/artio-joomsef.
P.s. also I hope your aware that the free version of JoomSEF puts spam in your "Generator" meta tag. Which can actually be bad for you SEO wise

For people not willing to spend the money on the paid version JoomSEF or for those on a tight budget I recommend using the free sh404SEF component (
extensions.joomla.org/component/option,c...k_id,2380/Itemid,35/).