Found the cause. $_SERVER HTTPS is empty even though HTTPS is being used. This causes the HTTPS check for the credit card form to fail. Even $_SERVER SERVER_PORT is reporting 80 instead of 443. Looks like you're behind a proxy or load balancer. Will need to improve the HTTPS check to also check HTTP_X_FORWARDED_PROTO.
I've patched your install with a fixed CBSubs build. I've confirmed the payment form works fine now. Am adding a bug ticket and fixing this issue for everyone else immediately.
forge.joomlapolis.com/issues/9214