Similar to this issue Can't setup Discourse ID: Application with this redirect URI already exists
My self-hosted Discourse has this exact error. But the thread above was already closed and didn’t see the answer on how to fix, but happy that issue was resolved. Is it possible to help my forum as well? Ready to share the URL in PM if it’s ok
Please share the URL via PM, thanks.
It seems that I don’t have PM permissions yet as a new user, it’s my first time interacting with Discourse. Could you PM me please, if it’s possible?
This should be resolved via PM now. The original issue was that this instance was registered a while ago, the server has issues, so the site manager had to reinstall Discourse and the credentials didn’t carry over.
I’m having this exact same issue. Can I PM you as well?
EDIT: nevermind I fixed it myself 
Could you share, how did you carry over the credentials? So that I could also do it myself in the future without bothering the Support?
The problem was with the domain, so I switched to a subdomain and it fixed it.