Why am I getting a 403 error when requesting a Token?

If you’re using the App Key from the development environment, your app can only register up to 100 users (to prevent accidentally using the test environment as the production environment for security reasons). Once this limit is reached, HTTP requests will return a 403 status code with an error code of 2007. You can delete test users in the Developer Console.

For detailed Server API error codes, refer to the documentation: Status Codes | RC Developer Documentation