Typically when we see this error and you're sure that your client key is correct (which the error message suggests it is), it is likely the result of using the wrong credentials on your server. The key / secret / app_id on the authentication server must match the key on the client.

To resolve this error you can you check:

1) The credentials haven't been hardcoded somewhere (even though the correct credentials are being set through a variable/configuration)
2) Check the link for your auth endpoint in your client code
3) Try logging the APP_ID and APP_KEY in your code to check the variables are being picked up correctly.