[Plugin: External Videos] No video is playing

I have installed the plugin and created a new page called “Youtube videos”. The videos shown up but when I click a video nothing happend. When I′m in the admin panel and attach a video to the page and “view” it, I always get a 404 error message.
https://www.phelps-twins.com/external-videos/harry-potter-actors-open-birmingham-teenage-cancer-unit/
I don′t know how to get the external-video link. Neither I can′t attach them to a post because the save all changes button is missing too.
The link to the video page https://www.phelps-twins.com/videos/
Thank you for helping me

https://www.ads-software.com/extend/plugins/external-videos/