Note: This article has the reference to the issue with the fix available:
"IP addresses registered in Plesk were not added to the Fail2ban whitelist."
- Plesk 12.5.30 Update 7 19 October 2015 (Linux)
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.