Ngnix - FastCGI sent in stderr: "PHP message: PHP Notice: Undefined variable

It's not an error, it's a notice.

The script /usr/share/nginx/html/admin-interface/login.php is accessing the variable $confMsg which does not exist at that point.

You can either change the error reporting level in php.ini (which has impact on other scripts too and you don't want to turn off notices..) or fix the wrong variable access in the script.

The second solution would be easier because you'd just had to initialize $confMsg = '';.