There are 2 other posts on the forum referencing this issue. One said to just remove the redirect uri, which may work but is not a viable solution for me because then responses would be sent to our production domain instead of my dev environment.
The other post mentioned that the datacenter URL may be incorrect and linked here: http://bullhorn.github.io/Data-Center-URLs/. I have tried all the US ones and they either result in the same error or don't respond at all. I don't know for sure what my datacenter is because when I log in to the app as described on that page, there is no cluster ID in the URL, it's just https://app.bullhornstaffing.com/. Upon investigating the network calls for that page, I THINK I'm in CLS42
I would start by taking a look at this article directly as it will walk you through step by step the process:
Regarding the data center I can look that up for you. What is your company name?