When using Citrix XenCenter 6.5, I was suddenly unable to connect to a XenServer instance running 6.2. The following was in the XenCenter Event log.
Failed to connect to foo.example.com The request was aborted: Could not create SSL/TLS secure channel.
This was not due solely to the version mismatch but, seemingly, to an update in XenCenter which forces stricter SSL checks. The only difference I found was that the XenServer instance I was unable to connect to had a cert containing an old IP address.
The first thing I did was update the parameters used to generate the SSL certificate
Then I was able to simply regenerate the certificate using the new parameters (and giving the resulting certificate the new IP address)
/opt/xensource/libexec/generate_ssl_cert /etc/xensource/xapi-ssl.pem $(hostname -f) && /etc/init.d/xapi start
Once completed, I was able to connect successfully.