This morning I found out my server was throwing out 502 Bad Gateway pages. I investigated promptly and googled around. It turned out one of my jails plesk-apache-badbot had banned my own IP Address. So after I moved it to the trusted IP lists everything went back to normal. You will get the following warnings in your error logs of your sites:
Basically the proxy tries to connect to apache but can't because the ip address is banned.
Connection refused) while connecting to upstream