This NDI connection problem seems to emerge when a Mac sends back a 169.254.* IP address over USB, which appears to be some kind of proxy/ad-hoc network port that NDI cannot use.
This is when the app shows a warning triangle, as it can see that there is an NDI server but it does not receive any video frames after connecting.
Figuring it out
About this Mac → System Report → Network has some information
Why does mDNSResponder returns the wrong IP Address? (