If you have an error retrieving your authentication token, it is probably because you're still using the oauth/v2 endpoint. As of mid September 2021 we will progressively stop V2 access (only V3 will be available). Check out how to proceed here: https://developer.orange.com/tech_guide/2-legged-oauth-2-v3/. If you have question on the service decommissioning, please use the Contact Us form below.
Check the FAQ of your API: the solution to your problem is perhaps already known. (To see your used API, Go on My apps and select the name of your subscribed API) If not, send us a request.