I am having the same issue:
My device:
Windows 10 Pro, 64 bit
My Evernote version:
10.54.4-win-ddl-public (3978)
Editor: v166.1.20643
Service: v1.66.1
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As of today, I cannot attach any file to any note. I've tried both small and large file sizes. I've tried JPG, PNG and PDF. I've tried adding them as "attachment" or as "image." No matter what I select, the image will display for a second or two, and then I see the message that I've attached here. It says "Unable to Upload. One or more attachments could not be uploaded. A copy has been saved to your local file system." I've tried creating new notes and adding attachments to them. I've tried adding attachments to existing notes that already had attachments, and existing notes that didn't have attachments. Same result, no matter what.