A website is not accessible with “504 Gateway Time-out”

Log files show:

# tail -n1 /var/log/httpd/error_log
[Tue Sep 04 19:11:38.300358 2018] [mpm_event:error] [pid 1192:tid 139861305055360] AH00484: server reached MaxRequestWorkers setting, consider raising the MaxRequestWorkers setting

/var/www/vhosts/domain.com/logs/error_log
[proxy_fcgi:error] [pid 50203:tid 139860761302784] (70007)The timeout specified has expired:

https://support.plesk.com/hc/en-us/articles/115000064929-A-website-is-not-accessible-with-504-Gateway-Time-out-The-timeout-specified-has-expired-client-203-0-113-2-54693-AH01075-Error-dispatching-request-to