Log.php Error

I got so many same notices;

A PHP Error was encountered
Severity: Notice
Message: Undefined index: �NFO
Filename: core/Log.php
Line Number: 180
Backtrace:
File: /var/www/html/index.php
Line: 319
Function: require_once

I'm using the latest version.(3.1.11). The problem started after switching to version 3.1.11.

@sergenk,

When you upgraded did you verify that you could update your CORE file directory? When you said switching to 3.1.11, from what version did you come from?