We have attempted REST REST API calls for the Cane Bay Partners sandbox that are returning error 603 "Access Denied" even when API User has correct permissions. We have checked through the admin interface and have confirmed that IP restrictions are not set. We believe we have set up the user correctly.
Not enough information here, please provide traces of the JSON you're sending and receiving and the exact URLs you're trying to access. You will want to redact the client_id, client_secret, and access_token of course, but please keep your report readable. Use the Advanced Editor's syntax highlighter to highlight the JSON (choose JavaScript from the language dropdown).
in my case the error was related with the token parameters I wanted to pull users but instead of sending the token using query parameters it has to be using header, I hope this helps