• is the file location writable by www-data/http?
  • does the directory exist all?

Also, check /config/generis/log.conf.php, you may have returned a simple array instead of the 2-depths required levels.

// Wrong
return array(
    'class'     => 'SingleFileAppender',
    'threshold' => 4,
    'file'      => '/tmp/myloglocation/',
    'format'    => '%m'
);

// Right
return array(
    array(
        'class'     => 'SingleFileAppender',
        'threshold' => 4,
        'file'      => '/tmp/myloglocation/',
        'format'    => '%m'
    )
);