Apache: Service Temporarily Unavailable The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

You have this error and:

• your backend servers work perfectly (they respond ok by their ip:port)
• your firewall is open for http/s port/s

Then maybe this can help you:

/usr/sbin/setsebool -P httpd_can_network_connect 1

Enjoy 😉