Cron job problem

Hi, I have problem wit cron job in cpanel as whmcs staff said, its server problem let me explain about it . I have whcms and I set cron.php on my cpanel and I'm administrator of this host here is discussion with whmcs staff . " I see that the cron.php script completes successfully when you run it manually - this proves that the script's code itself is good and working. However it always seems to stop at the 'Processing Invoice Reminder Notices' stage when triggered by the cron job on your server. Something must be killing the cron.php script before it can complete when being run as the corn job, this will likely be some kind of server configuration stopping long running scripts or some other security measure. This is something your server admin/hosting provider will need to investigate and resolve " I get this error in cron job activity it sends to my email
Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent in /home/***/public_html/support/includes/classes/WHMCS/Session.php on line 0

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent in /home/****/public_html/support/includes/classes/WHMCS/Session.php on line 0
How do it fix this ?