If you get an error something like this...
when you are trying to use SSL to connect to ActiveMQ then the "certificate_unknown" error shows on the broker when the client doesn't trust the broker's certificate. On the client, I would see an error as well: "No trusted certificate found".
Make sure that you exported the broker's certificate (step 2 in How do I use SSL) and imported it on the client into a truststore (step 4). If you did those, did you specify the javax.net.ssl.trustStore system property when you started your client VM?