Over night something must have happened cause all the problems are back.
No update. No restart etc in the meantime.
Windows Defender will have turned itself back on. You will need to Tame Windows Defender. There is a thread on that topic.
With the latest security threats in Windows and other applications, it is likely that all security software is clamping down a bit on any App that does a lot of communication on the network, and internet. Security software is still the most likely cause, but to stop the display of DLNA Renderers? That is a bit weird.
If all the DLNA Renderers appear in the MC Windows interface, on the MC Server, and you run Panel on the same PC, then I would expect it to see exactly the same DLNA Renderer list. Panel is just taking that information directly from the Server, rather than gathering that information itself, isn't it JRiver?
I see various DLNA issues. For example, I couldn't play to my Sony TV just now, until I rebooted the TV. Then I was able to use Panel to play to the TV, for now. The Sony TV is a bad DLNA Renderer. The Sony TV showed in Panel at the same time that it showed in MC. No waiting, once it was found in MC, which took a couple of minutes.
But your issue is very strange.