We've had a couple instances recently in Plone 5 sites that the various image scales stopped working. The first time this happened, I was able to resave all the Images (it was a small site), and that fixed the issue.
Now it's happening in another site (Plone 5.0.6), and resaving isn't working, plus this site has a lot more images.
This does not look like a permission problem but some problem at the ZODB level: "blob"s are maintained as files on the file system and apparently for one of the blobs referenced in the failing requests the corresponding file is missing.
Reasons can be various. The most natural one: the file was inadvertantly deleted. Potentially, you see also the effect of hardware problems or file system, ZODB or "copy-paste" errors.
Apparently, the image itself is still accessible -- i.e. the elementary information; what seems to be missing is something secondary. This means, you should be able to recover. I would carefully look at the traceback to find out what exactly was lost and determine from that how to get it back.
As in your case, the affected image was copied, you could delete the copy and retry the copy-and-paste operation. If the error should persist, many reasons from the list above could be excluded (e.g. file deleted, hardware problems, ...).
Thanks for the response. I was thinking that perhaps the scales for the image might have been lost. I've tried recreating the scales for any images in that directory downward, and the blob issue persists. As you suggest I'll dig through the traceback a bit deeper.