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.