So I have a massive Dropbox library - 30TiB and millions of files - and sometimes the Windows Dropbox client crashes and keeps crashing. I've figured out the easiest way to recover from this is to use the windows advanced options to create a system restore point. Fixed. Stupid but fixed.
You'll need "system protection" turned on for the volume with Dropbox on it.
Update: Turns out turning it on then off might work better.