2 years later...
I have a brand new Mac Mini M4, latest macOS Sequoia 15.5 and up-to-date Dropbox. After sleeping then Dropbox sometimes doesn't sync and I have to do exactly this -- manually locate DropboxFileProvider and Force Quit it. Synching immediately resumes. Nothing else that I've tried works. The Dropbox app claims that there are no sync issues and re-starting that does nothing.