"CERT_AUTH_REQUIRED" is returned if the cert login request is made but the presence of the certificate in the request isn't identified.
Please check that the certificate is still available uploaded to your Betfair account.
Please can you also try using the curl request example below and let us know if you receive a valid response.
curl -q -k --cert client-
.crt --key client-
-d "username=testuser&password=testpassword" -H "X-Application: curlCommandLineTest"