When shit goes sideways, you have no idea what is broken. Logging also seems mostly non-existent, at least via macOS' Console app.
Shit went sideways around 9/23 for many people across different platforms. It was definitely on the service side, but status.dropbox.com reported no problems. At some point on 9/26, someone fixed it, but no one was notified. Most people eventually re-tried, for the 100th time, of unlinking and re-linking their account. No system logs showing problems and no reports from Dropbox.
If it wasn't such an established service, i.e. widely supported by applications, I'd have ditched it long ago.