I have exactly the same issue having upgrade the Composer server and first Connection server. We only have two connection servers but not replicated, one does internal and the other handles 2-factor for external. I have upgrade the internal and cannot connect to the admin console or through the View client. I have seen this issue when I applied certificates early last year and that related to the client connection FQDN being different to the actual server hostname. I resolved this using a SAN within the certificate request. However, that can't be the same issue for me unless the new install is not able to use the certificate! I have had a response to another thread that references the 5.3 release notes and changes to how Security Servers handle certificates, but applying this change does not fix my issue :-(
Upgrade Connection Server from 5.2 to 5.3 No Longer Accepting Connections