Websites are not accessible after website settings are updated via Plesk: 502 Bad Gateway

Plesk for Linux kb: technical ABT: Group A

Applicable to:

In Tools & Settings > Apache Web Server, Apache restart interval value is set to 0.
After settings update via Plesk sites show periodically show 502 Bad Gateway error.

The records below can be found in /var/www/vhosts/system/example.com/logs/proxy_error_log :

2020-04-29 13:35:29 Error 203.0.113.2 28571#0: *6219628 connect() failed (111: Connection refused) while connecting to upstream nginx error
2020-04-29 13:38:24 Error 203.0.113.2 31784#0: *6220942 peer closed connection in SSL handshake (104: Connection reset by peer) while SSL handshaking to upstream nginx error
Also the following notification can be received by Watchdog:

The Web Server (Apache) service on host example.com is down. The problem was discovered on Jan 14, 2021 08:57 AM.

When the interval is set to 0, Apache web server restarts immediately to apply new domain settings. Apache restart causes websites unavailability.

Log into Plesk.
Open Tools & Settings > Apache Web Server.
Set up a non-zero Apache restart interval, for example 600 (interval is set in seconds).