Hmm, does this happen across devices? Try refreshing the page, checking your network connectivity and then adding your end screen animation & card in your videos. Keep us posted!
Make sure that the app/browser is up to date and try clearing your browser's cache and cookies with these steps: https://yt.be/help/Cko1 . That usually does the trick. You can also try it on a computer and see if it works there. Keep us posted.