If you look at the included php files, you will find that error text at the top of almost all of them. It is included in a die() function, in order to prevent a user from directly access a file - that is, the file must be called from osTicket, rather than viewed directly by the browser.
Do you get the message BEFORE or AFTER you log in? I think the file you need to look at is probably osTicket/include/client/login.inc.php - if you have not modified this file at all, I would try uploading it again, and see how that goes.
If you are not sure if this is the correct file, you can change line to from: if(!defined('OSTCLIENTINC')) die('Kwaheri');
to if(!defined('OSTCLIENTINC')) die('Correct File!!');