This is an SSL alert.
Alerts are sent from one party in the connection to the other party to inform them that they detected a problem. Typically these are fatal, and the first party then terminates the connection.
In this case Logstash is reporting that it received an alert from someone (probably beats, since that's the only input you have) that says "I don't trust your certificate".
Which means the issue here is that beats is not configured to trust the certificate that Logstash provides.
You probably need to modify the beats config to include ss.certificate_authorities