You're accessing your site through HTTPS, of course it's going to check for SSL. Even when using dummy test gateway it'll check for SSL if you've set it to use HTTPS in your CBSubs -> Settings -> Credit Cards configuration.
It's a misconfiguration on your server, SSL is not setup properly. SSL is REQUIRED in order for CBSubs to remain secure. Your host should be using at the least OpenSSL and should be configured appropriately. Generally this means your server (apache?) is listening to port 443, but doesn't know to do so with SSL.
Try access your site with the following link:
yoursite.com:443/
yoursite.com/
If you can access and view your site with the first URL, then your server is rendering plain text on 443.
If using apache you can try the following fix:
ports.conf file
Listen 80
Listen 443 https