After security is enabled, you must use the domain name, not the IP address. Security works ONLY with the domain name, and will not be found if it is not typed correctly.
For example, you might leave the "www" off of the domain name, and your checkout will not work. You need to make sure you have the correct domain name when making links for your site.
Below are a couple of incorrect examples of links on a site: http://12.34.567.890/index.html http://yourdomain.com/index.html
Here is an example of what the link should look like: http://www.yourdomain.com/index.html
