OK. Your demo site does not allow file uploads or linking of files..... I tried to troubleshoot this before posting a forum topic.
You say you have no idea what I have disabled or what parameters are set. Well, linking is obviously allowed in some capacity - I have the media link box available. I said in my post that I can link to a file directly using http://sampleurl/samplevideo.mp4
I have verified that linking behaviour has been allowed globally, via the CB plugin and tab settings.
As I mentioned there is NO specific error message shown to help narrow the problem down, the entry just does not submit and the box turns red indicating an error.
Recap: The linking behaviour is blocking youtube links, but other direct file links will work.
Does linking behaviour need to be enabled for all file types (Music, photos, etc)? Or should it only be for video? Or Files?