This problem has been solved.
Here are the factors:
1. About 7 months ago, I must have restricted RDP access to my remote IP address -- which is fixed.
2. Recently, my cable modem failed and I had to replace it.
3. I didn't realize that my external address had changed when the cable modem was replaced.
4. Because it was so long ago, I had also forgotten that I had restricted the remote address RDP access.
So, it appeared that suddenly, and for no obvious reason I was not able to connect (I hadn't tried connecting for a couple of weeks since changing the cable modem, so the hardware change was not that obvious a factor).
The solution was:
1. See the firewall settings by adding to the startup script, the following Powershell line:
netsh advfirewall firewall show rule name=all dir=in
and then looking at the Serial Console Output after reboot.
2. I noted that there was an IP address restriction on the TCP-IN rule for Remote Desktop.
3. I added the following line to the Powershell startup script:
netsh advfirewall firewall add rule name="Open Port 3389" dir=in action=allow protocol=TCP localport=3389
4. After a reboot, I was able to connect using RDP.
5. I removed the new rule in the firewall settings and removed the restriction to the 'Remote Desktop' inbound rule.