Thank you for your answer. The issue concerned the syntax I used to specify the URI of the video. By typing manually the path between double quotes, it works (whereas before I was using the URI automatically loaded by the application).
The video now starts correctly, however, the block doesn't wait and it there is a "start activity" in a following block, the video is automatically stopped. I used the delay block to avoid this issue, but the delay block causes other problems. Is it possible by any chance to view all the video without using the delay block?