403 and/or 404 errors when submitting a form are usually generated by an apache module called Mod Security. Almost all webhost providers have this enabled on their servers to protect their users. However some webhosts are using strict rules to prevent attacks so this is why it throws a these errors. If after adding ad codes like Adsense or using a URL as a parameter in the address, then this is the cause. You can contact your webhost to white-list your domain from using these rules.