Hi Giogio,
This code works well for me on PennController 1.8, 1.9 and 2.0.alpha. Do you have a defaultVideo.disable() anywhere before that by any chance? If not, the problem could come from the format of the video file. All browsers are supposed to support MP4 files, but note that having a .mp4 extension does not guarantee that the content of your file has indeed been encoded in MP4
Jeremy