PHP did not respond properly to the request.
As a result, Apache considers the PHP-FPM service to be unavailable.

Detailed information about this error is available in the app’s Apache error log at:

/srv/users/SYSUSER/log/APPNAME/APPNAME_apache.error.log

The root cause of this error is often slow PHP code.
You should disable any plugins the app is using and
check the app’s PHP error log and PHP slow request log located at:

/srv/users/SYSUSER/log/APPNAME/APPNAME_phpX.Y.error.log
/srv/users/SYSUSER/log/APPNAME/APPNAME_phpX.Y.slow.log