in chrome the console error is:
Refused to frame 'https://www.dropbox.com/' because an ancestor violates the following Content Security Policy directive: "frame-ancestors 'self'".
in firefox error on page:
Firefox Can’t Open This Page
To protect your security, www.dropbox.com will not allow Firefox to display the page if another site has embedded it. To see this page, you need to open it in a new window.
all domains have been added to app setting: Chooser / Saver / Embedder domains
no external domains have changed.
I have checked that the sharing permissions between videos that do work and videos that don't work are the same. There is no other obvious difference between the working/non working videos.