- Plesk for Linux
The following error occurs on an attempt to connect Plesk server to WHMCS:
FAILED: Error code: 0. Error message: Curl error:  Failed to connect to example.com port 8443: Connection refused.
Inbound or outbound firewall is blocking access to port 8443.
Make sure that port is opened both on Plesk server and on a WHMCS server.
On Plesk, Plesk Firewall component can be used. It is available under Tools & Settings > Plesk Firewall.