If you're using a VPN, check whether it has an option called split tunneling you may use to let Thunderbird bypass the VPN. If that option isn't available, you may have to disable the VPN in order to be able to use Thunderbird (or any other mail client, for that matter).

If you're not using a VPN, the problem must be something else. Go to the account Server Settings and click on the Advanced button. What does it show for Maximum number of server connections to cache? Set it to 5 if the current value is greater, or a lower value (e.g. 3) if it's 5 already.

Varying the server connection parameter did not solve the problem, but did help in the isolation. The system began showing other symptoms, mostly a low storage indication. The system had caught a virus which showed as several "wa_3rd_party_host_3..." processes. The Norton Power Eraser cleared, although it took several scrubbing iterations to recover the storage.

Glad to know, and thanks for the feedback.

Another thing to try that I should probably have mentioned and that would also have helped you identify the problem would be starting Windows in Safe Mode with Networking mode (you may have to connect your computer to the router with an Ethernet cable to do that, WiFi is disabled in Safe Mode):

https://support.microsoft.com/help/12376/windows-10-start-your-pc-in-safe-mode

This would have disabled the antivirus and helped you figure that could be the problem if Thunderbird was working fine that way.