It is trying to call include/class.sys.php
Check at the top of the file, there is a line which looks like this:
require_once('../main.inc.php');
This calls mail.inc.php, which defines various things. Check that it contains a line which looks like this:
require(INCLUDE_DIR.'class.sys.php'); //system loader & config & logger.
This calls the sys class.
Check that there is a file 'include/class.sys.php'.
Frankly, I would be surprised if all of these things are not there. Surely you would get more significant errors if they were missing, but they are the most obvious things I would check for.
Have you made any modifications to the code??
