Do this…
- Quit DEVONthink.
- Go to this URL: https://www.dropbox.com/account/connected_apps
- Click DEVONthink Sync and press the Disconnect button.
- Relaunch DEVONthink.
- Click on the sync location in DEVONthink’s Preferences > Sync. It should open the authentication screen.
- Allow access and let the machine switch back to DEVONthink.
- Quit and relaunch DEVONthink and see if the behavior recurs.