@YouTube, please, why do you keep ignoring everyone. FIX YOUR SITE! Why can some videos not add playlist endcards, but others CAN add that exact same one? I have 2 videos that have the exact same playlist card, but one works and the other says it's a private playlist (it's not).
Thanks for the info. Could you try clearing your browser's cache and cookies to see if that makes a difference? Here's how: https://goo.gl/U6TA4B . Keep us posted.