Have you checked if the credentials (client ID, client secret) and permissions are correctly configured ?
Avoid using 'Anonymous' as the authentication method; instead, use OAuth 2.0 or a similar scheme.
Check that the Azure AD application has the necessary API permissions for Dynamics 365 or other services involved. Admin consent may be required for these permissions. Also, ensure that the user or application has the correct security roles within Dynamics 365.
Verify that the request is being sent to the correct API endpoint and that any necessary firewall rules are configured. Tools like Postman or Fiddler can help trace and diagnose the HTTP request to identify misconfigurations..