Symptoms
The following error occurs on an attempt to connect Plesk server to WHMCS:
FAILED: Error code: 0. Error message: Curl error: [7] Failed to connect to example.com port 8443: Connection refused.
Cause
Inbound or outbound firewall is blocking access to port 8443.
Resolution
Open port 8443 on Plesk and WHMCS servers.
On Plesk server, Plesk Firewall component can be used. It is available under Tools & Settings > Plesk Firewall.