Strange problem with Zoom connectivity

[StevePatentAtto]

Very briefly: I have four Windows 10 PCs, all on exactly the same home network, connected to the same service (Verizon FIOS). Two of them will connect to Zoom. A third used to, until a few days ago, when it stopped. The fourth I'd never tested, but now I've checked it and it does not connect to Zoom either.

Details: On the two working computers, when I open the Zoom app I can log in, and I can setup and participate in video conferences perfectly normally. I can also go into any web browser on those machines and connect to the Zoom.us web page.

On the two non-working PCs -- one of which had been working just fine with Zoom until a few days ago -- the Zoom application now hangs when I try to log in. Further, no matter what browser I try, the browser page hangs when I try to connect to Zoom.us. However, I *can* connect to Zoom subdomains (such as this community forum).

As I say, on the one computer, this problem presented itself abruptly a few days ago after years of using Zoom with no problem. Solutions I have tried:

1. Uninstalling and reinstalling the Zoom app.

2. Checking the firewall for any problems, or anything at all, related to Zoom.

3. Adding a rule to the firewall to allow Zoom.us.

4. Rebooting the computers. Turning the computers off and restarting.

5. Clearing the computer's IP cache.

6. Turning off the modem and the router and restarting them.

7. I have my home wired for Ethernet. With one of the working computers, which is a laptop, I disconnected the external Ethernet port, and reconnected that port (with the same Ethernet cable that works with the laptop) to one of the non-working computers. That did not resolve the problem on the non-working computer.

8. Clearing the caches on the various browsers that I use (Edge, Chrome, Firefox, Brave, and one other I forget which).

9. I had been using Microsoft's firewall. I installed Norton's firewall, which did not fix the problem and did cause other problems, so I uninstalled it.

Any hints, clues, suggestions, theories?

Steve