Removing an image from file_field: Cannot access empty property in field\field.attach.inc on line 677

I have a node with a file-image field and several images. I'm removing one image, this works. But clicking 'save' gives this error:

(white screen)
Cannot access empty property in C:\Inetpub\wwwroot\123.net\modules\field\field.attach.inc on line 677

Next time I edit the node, the image thumbnail/preview is gone, but the image is still listed on node/edit. Trying to remove & save keeps showing above error.

More from the logs:
Unable to generate the derived image located at private://styles/thumbnail/private/upload/page/large/daviddawson_photocostinradu_cr_20071106_0084-613.jpg.
The image is gone, i.e. deleted from the file system, but not from the node/file table.

Also:
The file was not deleted, because it does not exist.
Obviously.

I temporarily solved it by manually tracking the file-node relationship in the database, and removing it there.