Unable to access Plesk GUI: Address family not supported by protocol

Symptoms 

• The Plesk panel is inaccessible with the error:
  

  CONFIG_TEXT: ERR_CONNECTION_REFUSED

• The sw-cp-server service is in the failed state with the error below:
  

  CONFIG_TEXT: sw-cp-serverd[642340]: nginx: [emerg] socket() [::]:8443 failed (97: Address family not supported by protocol)
  sw-cp-serverd[642340]: nginx: configuration file /etc/sw-cp-server/config test failed
  systemd[1]: sw-cp-server.service: Control process exited, code=exited status=1
  systemd[1]: sw-cp-server.service: Failed with result 'exit-code'.

• The IPv6 interface has been recently disabled and the server was rebooted.

Cause

The sw-cp-server service is still configured to listen on the IPv6 interface, while IPv6 interface does not exist on the server anymore.

Resolution 

1. Log into the server via SSH.
2. Rename the /etc/sw-cp-server/conf.d/ipv6_ports.inc configuration file:
   

   # mv /etc/sw-cp-server/conf.d/ipv6_ports.inc /etc/sw-cp-server/conf.d/ipv6_ports.inc.back

3. Execute the commands below one by one to restart sw-engine and sw-cp-server services:
   

   # systemctl restart sw-cp-server

   # systemctl restart sw-engine