Here’s the solution (at least on a Mac)
- Click on the Dropbox icon from your Desktop and quit the application.
- Open your Activity Monitor from your Applications folder.
- Force quit out of the instances of the Dropbox application that are running by searching for “Dropbox” in the search bar, then clicking on the “x” force quit icon on the top left.
- Re-start Dropbox from the Applications folder.
- Click on the Apple icon in the top left of your screen.
- Force quit and then restart Finder.