Some information in case someone else has the same problem.
I had problems with the video display in QGC under Ubuntu 22.04
After I changed permission, removed modem manager and installed gstreamer as described here the video display still did not work.
Solution was then to remove va-api
sudo apt remove gstreamer1.0-vaapi
Thank you! That was the same issue I had. Your solution worked!
wow! solved my problem. bravo!
I wonder if this would work for QGC in windows. I complied QGC (master) in windows 10 (VS2019) with QT6.6.3. Everything works well but the UDP video stream doesn/t show while the video source list is complete in General setting (meaning gstreamer is properly installed). Anyone knows?
Kudos! @divebubble great find!