To make matters simpler, let's try clearing the data manually and setup sync target one more time:
- Go to main Joplin menu > Open profile directory (it should be named
joplin-desktop)
- exit Joplin
- rename this profile directory to something else: for example
joplin-desktop > joplin-desktop1
- start Joplin
- setup your sync target (Dropbox) and start synchronization
- the notes should start appearing