Can't save on video slide

Error:
  • A slide was created with the "Add video slide" button, and you can't save any changes made to this slider.
This troubleshooting is only for video slides. If you can't save on any slide, check out this instead.
Cause:

When you are creating a Youtube or Vimeo video slide, then our system puts that video's thumbnail into our thumbnail option. This is linking to an external website, and your problem is caused by your server, that some servers doesn't allow you to save down external urls, so you should just remove this thumbnail, or replace it with an image from your website, and it should be fine.