I just had the similar situation "Bad or unknown responce from aaa test server" when we do aaa test server and NPS server security logs shows "An Access-Request message was received from RADIUS client (ip addr) with a Message-Authenticator attribute that is not valid.
This could happen due to any Network corruption, latency, or other network problems unrelated to NPS/IAS/others might produce this condition.
Kindly make sure that the remote RADIUS server configuration, including the IP address of the RADIUS client/proxy server and the shared secret configured on the server running NPS and on the RADIUS client, is accurate.
encrypt disable
show aaa authentication-server radius <SERVER-NAME>
above commands will show you the sever config with the key, IP address, ports etc and please make sure all is correct.
This fixed the issue for one of my customer.
Thanks</SERVER-NAME>