Hi;
All PHP-FPM versions between 5.3-7.3 are active. However, a site running on 5.6 periodically:
"Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later. "
Gives a fault.
I think this error was caused by FPM communication errors. FPM repeatedly gives error when closing version 5.6; however, when FPM 5.6 is active, F5 on the site gives "Service Unavialable" error periodically.
https://medicalli.com/ -> Make fast F5 and observe the error.