The "Service Unavailable" error on CloudLinux servers occurs when the server cannot handle the request. It may be due to overload, configuration issues, or resource shortages. Fortunately, there are several possible solutions, such as adjusting server settings, optimizing resources, or using the following instructions.
When restoring an account or restarting the server with CloudLinux, a website may display a "Service Unavailable" notice, and it's very likely that the following error can also be found in /usr/local/apache/logs/error_log
$ grep -i cpanel_user /usr/local/apache/logs/error_log
[Sat Jun 06 13:22:16.877304 2020] [lsapi:error] [pid 23113:tid 47912125892352] [client 187.188.127.42:46658] mod_lsapi: [host fusioncelular.mx] [req GET / HTTP/1.1] Connect to backend failed: connect to lsphp failed: 110Solution for an individual user
> cagefsctl -m cpanel_user
Solution for the entire server if all sites are failing
> cagefsctl --force-update> cagefsctl -M