Our users have this problem occur regularly. When they are looking at images stored on the server they will see thumbnails of the image, which is what they want. After a while, can be several days, the images lose the thumbnails and are replaced with the generic image icon. Restarting the server brings back the thumbnails.
The thumbnails disappear for everyone at the same time, so it seems to be a server related issue.
Why does this happen and how can we fix it?