You either have issues with the transport security (encryption) or the connection gets blocked somewhere (Core PC, Router). This does not look like a Roon issue to me, so keep troubleshooting your (network) side of things.
Try disabling all IPv4 firewalls for troubleshooting (NAT should still protect you during this time), check your hosts file on the Core PC, … . Only you know what else you might running that might block the connection.
Can you login to the forum here from the browser on your Core PC running Windows 10?
Nope, couldn’t login to the forum from Core PC. Will keep troubleshooting router/network.
Is this handshake/authentication running on other ports than 443?
Would you say that if I can logon to the forum from a machine on the same subnet, then it would most likely be something on the Core machine rather than on the router?
Not sure when I’ve been playing with the host file, but I obviously have. Thanks for the reminder. I will have f@ckt up a lot before that had popped into my head.