HTTP 200 Response with IP_NOT_ALLOWED when trying to refresh OAuth Token

HTTP 200 Response with IP_NOT_ALLOWED when trying to refresh OAuth Token

Our CRM API integration, (V2 API), has started getting responses from the refresh endpoint where the body is just:

{"error":"IP_NOT_ALLOWED"}

With an HTTP 200 Response Code.

- We haven't changed the IP address making requests.
- API integration has been working for over a year with no problem.
- We don't use any IP restrictions anywhere.

Does anyone know what is happening, and how to solve it? Customer data isn't getting into CRM like it should be.