Oh, you are using OSTicket Awesome 1.17.
If you want to use Oauth2 please update to the latest release.
The osTicket developers made a bunch of changes to Oauth2 in 1.17, and then ran into a bunch of glitches. 1.17.1 and 1.17.2 were primarily bug fixes for these very types of issues.
I am confident that updating to the latest code with sort this out for you. So I’ll mark this as ‘Resolved’ for now. But you can still reply if you run into any more trouble.