To make matters simpler, let's try clearing the data manually and setup sync target one more time:

  1. Go to main Joplin menu > Open profile directory (it should be named joplin-desktop)
  2. exit Joplin
  3. rename this profile directory to something else: for example joplin-desktop > joplin-desktop1
  4. start Joplin
  5. setup your sync target (Dropbox) and start synchronization
  6. the notes should start appearing