I found that I can start a YouTube video in stock YouTube player this way:
Intent i = new Intent(Intent.ACTION_VIEW, Uri.parse(\"vnd.youtube:U79wVTdnMpw\"));
I found the answer on this Google product forums which helped to solve my problem - which was exactly the problem you're experiencing :)
SOLUTION: Give ONLY the YouTube ID and not the entire YouTube URL to the loadVideo() function.
loadVideo()