I'm having a similar issue, but get a slightly more detailed error message:
:message=>"All hosts unavailable, sleeping", :hosts=>["xxx.xxx.xxx.xxx"],
:e=>OpenSSL::SSL::SSLError: certificate verify failed>
What is strange in my case, is logstash-fowarder set to use the same CA certificate works just fine. I also used openssl to verify that the ca certificate I'm using does verify the certificate using by the lumberjack-input.
I know the hosts can communicate with each other, since logstash-forwarder works, just not logstash with the lumberjack-output plugin.