While using Nginx as a reverse-proxy, IP address of the server, not client is written to the Apache server log. And
PHP $_SERVER["REMOTE_ADDR"] function returns wrong output. Therefore AWStat statistics is not properly calculated.
The issue is considered to be a software issue with id #PPPM-2144 .
The issue has been fixed in Plesk 12.0.18 MU#24 . Please install all the microupdates.
For Plesk 12.5 refer article #213376349