An error occurred while trying to upload the attachment(s)

When I sync my library, I am getting this error message: “An error occurred while trying to upload the attachment(s)”. This has been consistent now for some months. New references do upload, but without the attachment. This is problematic in that I don’t have my newer attachments online AND the two versions of my library (on my home and work computers) are not equivalent.

I found a post chain from a few years ago suggesting archiving my library, deleting all the references in the online library, uncompressing my library and syncing it. Apparently this worked for that user. However, I have 7500 references and over 7GB of attachments. Living in South Africa means a slow internet connection - it would take several days to perform this upload, which I’m not keen to do.

Any suggestions as to what caused the problem and how to resolve it?