A System Restore is probably the easiest, but it will mean redoing lots of work and the problem may happen again.
This is what I've ended up doing and it has resolved the issue. We'll see if it returns but, for now, I have MC27 with library server running on port 52199 using a restore of my previous library and settings with Windows Defender and Windows Firewall both running. It's as if nothing ever happened.
After all that troubleshooting, would have been nice to pin down a cause and avoid redoing some setup, but will just have to chalk it up as one of those unexplained things that goes wrong proximal to MC from time to time.
Roderick- thanks for stepping in and I've answered your remaining suggestions so they are available in the record for searches if anyone else is unlucky enough to run into this problem as well.
You could try running WireShark to see if there is any activity on Port 52199.
I guess that could be useful if I needed to log activity over time, but I was already monitoring that port with a handful of other similar tools and there was never anything running in a port number even close to 52199 at the same time I was trying to start the server.
You could also uninstall MC27, then find the JRiver trees in the Registry and delete them, then reinstall MC27 and Restore a MC26 Library Backup... from just before you installed MC27. If that works but would mean you lose some work you have done in MC27 (tagging or whatever), then you could Restore your most recent MC27 Library Backup but without settings. You may need to tweak some settings after that, since there have been some changes and the defaults would be put in place.
I did do something like that. Full uninstall deleting registry and started with clean MC- no library, no settings. Went straight to server settings and ran the test and it failed.
Can you connect to the MC Server from a Client using the IP Address of the MC Server? i.e. Add a Library and use a URL such as 192.168.0.xx:52199.
For that matter, does Panel work, even on the MC Server PC?
Prior to the system restore fix, the server wouldn't start at all on 52199, so couldn't connect to the library from Panel, MC Server, Gizmo, etc. Could see it wasn't running in windows services and that port was empty. Now that it's running, can see MC on 52101-4 and 52199 with associated connections.
You can test the Access Key lookup on the JRiver servers using http://webplay.jriver.com/libraryserver/lookup?id={Your Access Key}. Does that work, and show valid information?
Yes. And that was expected because the key always worked fine if the server was running on a different port. However, when I did a full uninstall (registry and all) and started with a clean install, no library, I received a different key and it also did not work on port 52199.
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Disabling antivirus sometimes isn't enough. Uninstall it. Configure Windows Defender as advised on the forum.
Windows Defender is integrated into the operating system, so can't be completely "uninstalled"- but all functionality was
fully disabled (when not disabled, Defender is configured as per the forums and MC 27 was immediately added to exceptions upon install).