So I think I figured out the issue. It seems to be that Excel will only open files with file names under 216 characters, so if a file is embedded in many sub-folders with long names, the file name will easily exceed that limit. Obviously, this largely Excel's fault especially since the error says the file has been deleted or moved instead of simply notifying the name is too long. But Dropbox is also to blame. Since Dropbox won't share sub folders separately from the parent, we end up using very long parent and sub folder names.
... View more