Sites are unavailable with HTTP 503 error: failed to make connection to backend:

[Suhitha]

Question: Sites are unavailable with HTTP 503 error: failed to make connection to backend: httpd-UDS


Symptoms

Websites hosted on the Plesk server are returning an HTTP 503 Service Unavailable error.
There are the following records in /var/www/vhosts/example.com/logs/error_log file:
[proxy:error] [pid 4322:tid 139839798191872] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /var/www/vhosts/system/example.com/php-fpm.sock (*) failed
[proxy_fcgi:error] [pid 4322:tid 139839798191872] [client 203.0.113.2:58526] AH01079: failed to make connection to backend: httpd-UDS


Cause

Unclear.


Resolution

1. Connect to the server via SSH.
2. Run the following command:

# plesk repair web