I faced the same problem today, and for me the error in the logs was:
W/YouTubeAndroidPlayerAPI﹕ YouTube video playback stopped due to unauthorized overlay on top of player. The YouTubePlayerView is not contained inside its ancestor com.google.android.youtube.player.YouTubePlayerView{42686bc8 V.E..... ........ 0,0-1200,675 #7f0a00a0 app:id/video_player}. The distances between the ancestor's edges and that of the YouTubePlayerView is: left: -10, top: -10, right: -10, bottom: -10 (these should all be positive).
I fixed this by removing the padding in the YouTubePlayerView in the layout. So my layout looks like this:
<com.google.android.youtube.player.YouTubePlayerView
android:id="@+id/video_player"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#000" />