This issue was posted here like five years ago by a different user and the fix worked for them but I've encountered the issue now and the fix isn't working.
Basically, I've got a new Mac, synced all my files from my old Mac but they're mostly showing up as Zero Bytes and they're not openable. I tried going to my preferences->Account->holding down the alt/option key and clicking "Fix Permissions". It prompts me for my computer password, I enter it, it seems to work but doesn't. So I do it again, then I receive the error message in Dropbox: "Dropbox was unable to fix permissions in your Dropbox. Please contact support if the problem persists." I'm on the basic plan, so I don't have access to proper support (bad show, guys) so I'm hoping someone can help.