Can you please send the screenshot of the ‘Connector’ stack trace,
It will help us understand where the issue is.
According to the Exact API documentation and the Postman collection, you should also add the redirect_uri parameter
The Postman collection (Postman is the Swiss army knife for REST API testing)
Go Make It
Update: found a bug.
The issue was that somewhere in my microflow, the refresh_token got truncated (at this moment not sure why). Apparently the API gives in this case an 400 back, and not a 40 (unauthorised).
Thanks for thinking along