Dropbox has changed its policy for free tier accounts. Only three devices may be linked to the account. This means if you use Dropbox’s free service to sync Joplin notes you can only access the notes from three devices.
Perhaps the Joplin team can expand the number of sync services to include a few more as Dropbox is not so attractive as it was. I have just discovered sync.com and it looks good. They allow unlimited devices, just like Dropbox used to.