You must configure your "/log" folder to a absolute links of your server, in the backend of joomla. For example : /www/public_html/log.
create a .php file and paste this code to know what is the absolute path to your server. Upload and call it on your browser: ex: www.yourdomaine.com/path.php . this
this is the code
<?php
$path = getcwd();
echo "Your Absolute Path is:" . $path;
?>
$path = getcwd(); echo "Your Absoluthe Path is: "; echo $path; ?>