Problems calling cron.php

We recently spotted an error returned from cron.php

PHP Warning:  include_once(): Unable to access ./includes/bootstrap.inc
in /var/www/vhosts/.../httpdocs/cron.php on line 9

The following tweak helped: Create a new file "cron-tweak.php" and point the crontask towards it. Insert the following lines:

<?PHP

$doc_root = dirname($_SERVER["SCRIPT_FILENAME"]);
chdir($doc_root);

include_once './cron-ori.php';
print "cron completed";
?>