Update: I have excluded, some time ago, the synchronization of .lock, .sync, locks and temp in Dropbox/Applications/Joplin. (With the Dropbox application itself, not possible on the Web, AFAIK.)
I restored .lock and .sync, but I still had the same error.
I restored the two others, and synchronization went without error.
I fear the advice given above is no longer valid, and we have to live with permanent creation and deletion of files there.
I disabled the notifications of Dropbox to avoid seeing them all the time.