Video Thumbnail not HTTPS (with Flash player)
As seen here: http://support.jwplayer.com/customer/portal/questions/11381246-video-thumbnail-not-https, there was a problem where If you didn't specify an image thumbnail when embedding a youtube video jwplayer was serving an image from an http URL even though both the YouTube video URL and the site URL are both https.
The problem has been fixed for the html5 player but not for the Flash player. The change needed is most probably in \src\flash\com\longtailvideo\jwplayer\model\Playlist.as line 114. This is causing https issues that are very annoying for the users of the site.