Make sure the path to your "log" and "tmp" folders are set correctly in your global configuration.
To find this path, copy this code into a text editor, save the file as absolutepath.php and upload this file with a FTP client to your root folder.
<?php
$path = getcwd();
echo "Your absolute path is: ";
echo $path;
?>
In your browser, type in:
www.yoursite/absolutepath.php
and you will get the absolute path. Modify accordingly the two lines in configuration.php. For example, it could be:
var $log_path = '/home/yoursite.com/public_html/log';
var $tmp_path = '/home/yoursite.com/public_html/tmp';