I'm having the same issue with my Ultimaker 3 on the latest firmware (4.3.3.20180529). The camera feed works if I open the feed directly (http://192.168.1.119:8080/?action=stream).
It does not work via the Ultimaker App or Cura (3.6). When I open the printers web page directly (http://192.168.1.119/print_jobs) the video is just black. Inspecting the link shows the issue is that it is trying to open src="http://127.0.0.1:8080/?action=stream" which is using the wrong IP address.
This seems like a bug in the printer firmware given that the direct link printer isn't working correctly.