After enabling nginx on server, all websites are showing 502 Bad Gateway error.
Fail2ban is installed on server.
Caused by internal issue #PPP-18023 . Issue was fixed in Plesk 12.5 MU#7 .
Apply latest Plesk microupdates .
In Plesk > Tools & Settings > IP Addresses click Re-read IP button to add server IP addresses to Trusted list of Fail2Ban.