Have you upgraded PHP recently? Or changed your php.ini?
I would check and see if you have session.auto_start = 0
set in your php.ini. If this was on it would explain why you were getting that warning.
Alternatively you could always suppress the error by editing /home/pos/public_html/helpdesk/main.inc.php on line 35. And changing:
ini_set('session.use_trans_sid', 0);
to
@ini_set('session.use_trans_sid', 0);
And while it will suppress the error, it doesn't mean that it will actually make the page work. Something somewhere is starting the session before it should be. BTW, you didn't really mention which version of osTicket you are using.