There could be any number of things wrong. The correct way to trouble shoot this is to change one variable at a time and keep testing to figure out which "thing" is causing the problem. Once you know which "thing" is causing the problem, then you will be a lot closer to getting it fixed.
Try changing the name of the file from main.php to mail.php-old but still request main.php in your browser and see what the results are.
Try changing include_once "./include/cb_top.inc.php";
to
include_once ("./include/cb_top.inc.php");
and see what the results are.
Try removing this line from your http.conf:
Options +Includes
Confirm that a simple PHP script works. Make a file called test.php and it's entire contents should be this:
<?php echo 5+1; ?>
Once you are certain that PHP is working, and you are certain that server side includes are not complicating the problem, and you know what requested URL the error message is referring to, you will be much closer to solving the problem.