After ample consideration, I'm now pretty convinced that the reason Redrock 12 didn't succeed in correctly posting his video must have been that he probably skipped a crucial preliminary step which most people know about and take for granted.
In order to post any video (or other media from the list) you first have to activate the video itself in a new tab or window (whatever you prefer and habitually choose). Doing that ensures that you single out only one video at any time and will have the correct address of it (URL) in the address bar for copy/pasting. This also prevents you from unwittingly embedding playlist like sequences of multiple videos without intending to do so, as could have happened with your Cat Stevens video.
As for copy/pasting itself, the preferred method would be to right-click the address bar at an empty space of it, choose copy in the drop down menu and then pasting it in the Insert media thingy within your post box. This will guarantee a flawless transfer whereas manually highlighting and copy/pasting would be more prone to making mistakes, which could also be the sort of mistake that crept in while trying to post your video, I presume.
In terms of the Mozart example:
[MEDIA} https: //www.youtube.com/watch?v=Rb0UmrCXxVA [/MEDIA]
the suffix/extension in red has to be included, has to be complete, and has to be without any flaw. Otherwise, your video cannot and will not play.