There is a very good chance your host has installed a add-on for apache called mod_security.
Make or edit your forum .htaccess file and add the following code
<IfModule mod_security.c> SecFilterEngine Off SecFilterScanPOST Off </IfModule>
Should this not work, or you get a 500 Internal Security Error, your host has made it so you're not allowed to make these changes. At this point contact your webhost and ask them to re-tune their mod_security rules. They should be able to look at their logs, find the triggers and make adjustments.