Hi there,
As a first pass I’d be tempted to direct your integration to a service such as Postbin and repeat your refresh request. This service allows you to see the request that’s received by the server, which should allow you to spot any difference(s) between our documentation and the request being sent by your code.
If you’re still having trouble, feel free to post back here, though please could you share all of the information outlined in this post so we can more easily find your request(s) in our logs:
Kind regards,
Dave J