Thank you much for your reply. What you describe does not happen, that is exactly the problem.
I was not working with urls / youtube.
I have uploaded a mp4 video from my hard drive (exactly according to the manual requirements, 640 x 360) and added it to a plaque. The video autoplays vertically, but the image doesn't rotate to full screen horizontal when you rotate the phone (in my case iPhone 5s).
Any idea what could be the cause?