Disable Apache mod_security, search the forum for "mod_security" for more info

I don't have direct control over that setting from my host provider, but I can select which version of PHP my site uses. I changed from 5.3 to 5.2 and that seemed to solve the issue. I don't know if it has to do with the PHP version, or with my host's settings files having different settings. Thanks for your help.

It looks like mod_security is not enabled in PHP 5.2 then. Anyway, v5.2 is outdated, you should better work with hoster to disable mod_security for your account.

You can also try disabling it in .htaccess file, google for "how to disable mod_security"

I'll talk with the host - it's not an Apache server, so I don't have th .htacess option. Thanks.