@fareast55: Must be parallel universe to the one I'm living in 😉
Support wrote back that everything is resolved. After clearing the browser cache, the message ""App Video Preview Error, Some error has occured with your app video preview" had disappeared. Yesterday, my app got approved and is now in the App Store. I checked again this morning in iTunes connect, and the error message was back again. However, the good news is, the preview is shown correctly in the app store 🙂 So this problem apparently just has something to do with the iTunes web front end, not how the video is stored on the server.
I just checked again now, and the error is gone. Looks like the system having some last hickups 😉 So for everyone still seeing the error message, try a couple of times and see if it is revolved.