We assume that you are using one of the built-in payment gateway extensions (Stripe or PayPal). If not, contact developers of custom payment gateway extension that you are having problems with.
Subscriptio uses standard WordPress function wp_remote_post() to communicate with remote servers. This function supports directive sslverify (default value is true) which in turn controls CURLOPT_SSL_VERIFYPEER and CURLOPT_SSL_VERIFYHOST.
Subscriptio had sslverify disabled in earlier versions. This allowed wp_remote_post() to communicate with remote hosts over SSL even on servers with missing CA certificates, however, made the system prone to "man in the middle" attacks. In version 2.1.1 sslverify was set back to true. Security was improved, however, this caused problems on servers with incomplete configuration (missing CA certificates).
The best way to solve this is to contact your hosting service provider or server administrator and ask them to set up required CA certificates.
Another option (less secure) available since version 2.2.1 is to override sslverify value using a filter hook subscriptio_sslverify. Example usage:
function my_sslverify_override() {
return false;
}
add_filter('subscriptio_sslverify', 'my_sslverify_override');
If this does not help, be sure to get in touch with our support team.