Images do not load.

Images do not load.

I have added some pictures to a note via the android app.
The pictures were uploaded successfully. Via the browser I was able to view the note including pictures. 
Up to this point, everything was fine.

However, after some time, some pictures are no longer displayed in the note. Only a loading circle is displayed via the browser and in the Android app → see attachment.

My questions:
 - How could this condition occur?
 - Can the pictures be restored?

I hope the pictures have not been lost.

Picture not shown in Android app