I had this issue recently when trying to bring in NDI Desktop Captures of pinned Zoom participants.

This had worked fine for me for a few years and then suddenly about a month ago it didn't work. I had the momentary audio and then none. I can't remember if the video capture continued or not.

I tried this on two different PCs and had the same issue on both. Note: The host PC had Windows 11, and one of the PCs with NDI Desktop Capture was running Windows 10 and the other with NDI Desktop Capture was running Windows 11 (upgraded from Windows 10). Not sure if that had anything to do with it.

I checked to ensure all Windows updates were up to date and rebooted them, and I then updated to NDI 6. No luck.

Not sure if it has anything relevant to your experience.

In my case, I ended up using the new Zoom plugin in vMix 27.

I often had the same problems and then found the following solution. (All my systems are running WIN10)
If you haven't installed the NDI tools yet, download and install them. I don't know if it is important that vMix uses the same version of the NDI DLL as vMix. I always use the latest NDI Tools version.
Enter all your IP addresses in the NDI Access Manager of the NDI Tools, under the tab “external sources”, click on + and add the IP's of your NDI sources and vMix Computers in your local network. This also works for me with Zerotier addresses. As soon as the IP's were entered in NDI Access Manager, vMix soon found them. This behavior was the same with various vMix systems in different networks. If a vMix system did not see an NDI source, adding it in the NDI Access Manager solved the problem. I don't know why this is the case. Mainly because, according to vMix, it is not actually necessary to install the NDI tools. The way in which vMix finds the NDI sources or displays them as input must be different from other systems. In almost all cases where vMix did not display the NDI source, NDI's Studio Monitor app found it, but not in all cases. This may also have something to do with the network setup, but since this problem always occurred in the same way on different systems with different networks, this is just a guess.
recommended ndi settings for network switches
If you have a managed switch, check the recommended Network Switch settings for NDI (search the internet for "recommended ndi settings for network")
Well, good luck and I hope that this solution works for some of you as it does for me.

I recently started having a similar problem. I had stumbled on adding the cameras into Access Manager which then made the cameras "live" in vMix. What's strange is vMix saw the cameras from before but the video was completely black. I did install NDI Tools 6.x and copied in the latest NDI driver as stated above. This was part of my PTZoptics camera configuration change from 1080p 30fps to 1080p 60fps. This seemed to correspond with a ton of Defender CPU usage. I resolved that by excluding the folders for defender, vmix. I also excluded the vmix, ndi, and defender binaries. Along with some other tweaks in the cameras and vMix. Very strange. I only have 2 cameras over NDI using a Dell Alienware 15p laptop.