Hi, I have tested Erel's example with "SuportedOrientations: portrait" mode and the video in full screen and I don't get any errors.
When you have the phone in portrait, and want to play the video, your app goes landscape (I guess that the api does make some kind of exception on playing in fullscreen and having the portrait mode blocked), so according to the activity lifecycle, it goes to pause, create and resume (theregoes why your app restarts). And when the video stops, the app return to your activity on portrait mode, doing the create and resume again.
So you should make sure that your Sub Activity_Create is using the FirstTime parameter to initialize just one time the variables that need so, and don't loose the state when restarting the app. So basically you need to save the state in the pause Sub, and restore it in between Create and Resume