"502 Bad Gateway" on first request in a setup with Apache 2.4-servers as upstreams

[hheiko]

I've noticed the same problem between Nginx Proxy (Win) and CentOS based
Apache 2.4 Backends. So I finally changed all backends to nginx+php-fpm...

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,268306,269500#msg-269500