This isn't a bug with 1SE but a well-known industry problem with iCloud Photo. We're trying our best to address it but it's a moving target.
The best workarounds are to log out/log back into iCloud, download the video locally in your Photos app, or check your device for available storage.
You can read more on the documented issue here: https://www.imobie.com/support/how-to-fix-error-downloading-photos-from-icloud-photo-library.htm
Sorry this is happening!