Image occlusion cards do not work

Does anyone know how to get pictures back? Some of my image occlusion cards have the bug that they can no longer be opened. Can anyone help?
Or do you know how to avoid this in the future?

Yes, they are and it worked in the past. And I also checked the storage pathway regarding this “???” → “null” error but everything seems normal.
Fortunately, the problem “only” occurs in the desktop version, but it’s still quite annoying, because that’s how I normally study.

Are you sure they were uploaded to remnote servers or to a reliable host (Imgur, for example)? Could be the original image’s addresses are no longer reachable.