Mod_security helps protect you against various Perl, PHP and Ruby exploits but it can have false positives depending on the URL the user is visiting. If you're receiving a 403 forbidden on a page it is most likely mod_security. You can disable mod_security by adding the following to your .htaccess file:
Once disabled make sure to keep your scripts up to date as there is no prevention system if there was ever to be an exploit for the script.
UPDATE: It seems that Mod Security 2.5+ doesn't let you use the htaccess file to disable it If you need to disable it for a file or directory, please open a support ticket. We will only disable it for a file or directory.