@Rémi Denis-Courmont I didn't know that. I had taken the modified (by Casketizer) youtube.lua and renamed it as youtube.luac, since that's its name in Linux. That was my error, renaming it, but now knowing what you told me I kept it as a "lua" file and it's working now.
BUT, I have another problem now: I have a slow internet connection, so I always watch my videos in 360p. So, when I open with VLC a YouTube video or create a playlist (.m3u file) with YouTube videos in it, I always add "&fmt=34" to the URL to force VLC to show me the videos in 360p resolution. And now if, for example, I try to open
http://www.youtube.com/watch?v=ZsJx_RWXx3I and add &fmt=34 at the end, like this:
http://www.youtube.com/watch?v=ZsJx_RWXx3I&fmt=34 this error appears: "VLC is unable to open the MRL «
http://youtube.com/get_video?video_id=Z ... vg=&fmt=34".
But if I try to open another video in 360p:
http://www.youtube.com/watch?v=-y68iWtU49w&fmt=34 it plays Ok.
Well, thanks to you and everyone for all your help.