Now that you have upgraded to 1.6ST you should be able to download and install and upgrade using 1.7.1 or 1.7.1.1. If you have problems with that 1.7 is available here:
https://github.com/osTicket/osTicket-1.7/tree/1.7.0(https://github.com/osTicket/osTicket-1.7/tree/1.7.0)
https://github.com/osTicket/osTicket-1.7/tree/v1.7.1(https://github.com/osTicket/osTicket-1.7/tree/v1.7.1)
https://github.com/osTicket/osTicket-1.7/tree/v1.7.1.1(https://github.com/osTicket/osTicket-1.7/tree/v1.7.1.1)