My problem turned out to be related to the fact my server went from wireless to ethernet. As it turned out, that new connection was defined as a public network. Once I corrected it to be a private network, the issue resolved.
In researching this, another common issue is when a VPN is active.
Check to make sure your PMS connection to your network is defined as “private”. Also, turn off all VPNs at least long enough to eliminate that as the source of the problem. Good luck. I know how frustrating this gets.