I had similar issues, the test server was working but the production server would not, so I had done this test and it helped me fix my issue.
If you like to test this, create a new folder within the same server where your current osTicket is installed, for example:
let's say your current install is on: /www/osTicket Then create a folder in it called Test /www/osTicket/Test
Then download a fresh copy of the new osTicket from osticket.com/download version 1.17.2 Go through a setup like it's a new install with a new database and for this Test to keep it live server and Test separate
You can create new app registration for this Test to keep it live server and Test separate
If this works then something else is not right