Roundcube needs to call back to your control panel domain to authenticate the user and get the IP of the assigned mail server. Check public_html/config/config.inc.php, it should be pointing to your main control panel domain. If the control panel domain has a WAF or external proxy (like Cloudflare) they might be blocking the request, you should whitelist the control panel server's IP.
The per-server webmail accessible on mail.customerdomain.com does not require a callback.
