I've seen this before and it is either caused by trying to login and view a ticket that you aren't allowed to view or a ticket that has been deleted. Make sure you have permission to view the ticket that you are trying to view. For example if you are logging in by going to www.your-address.com/osticket/scp/tickets.php?id=18(www.your-address.com/osticket/scp/tickets.php?id=18) and that ticket is in the Support Department, you have to make sure that the group you belong to has access to the Support Department. I think this was my problem. Either that or ticket id 18 didn't exist.
