If the Endpoint and the Server suddenly cannot communicate after working fine for a while, suspect problems with any new firewalls, routers, antivirus software, anti-adware, or any other changes to the network or computers that took place at about the same time the problem started.

This can include upgrades to Windows or any software running on either the Endpoint or the Server (manual or automatic,) especially upgrades to software firewalls. It can also include changes made manually to the Windows firewall settings or the settings of any other device or software designed to block Internet traffic.

Also look for issues caused by computers (Endpoint or server) that have rebooted recently.