ramrajone I've gone through the documentation and set up Oauth at least 4 times, following the instructions exactly.
As it happens, your post was one of the top results I found on Google when researching this topic. I set up a separate App registration for the mailbox Oauth using your guide - again, multiple times.
Still no go.
I'm beginning to think that our firewall is doing something stupid, but our firewall logs don't show anything being blocked, and we use Oauth/SSO for many other applications without issue. Given that most others don't seem to have an issue, I'm starting to think that it's not an osTicket issue. I remain open to all possibilities, though.