Android cannot acquire sync lock

Each Joplin instance, when syncing, locks others out in order to avoid everyone changing everything at the same time. Technically, this happens by creating a 'lock' file on the server, in a 'Locks' folder. On Dropbox this is located at:
https: // (I intentionally added a space to avoid my post showing the Dropbox banner instead of the url)
Connect to Dropbox with your bowser, go there. If this folder is not empty, nobody can sync until that lock file is deleted.
Now it may happen that you have quit or disconnected a Joplin instance without allowing it to finish the sync : wifi link loss, computer put to sleep, whatever. In this case the lock file stays there and just nobody can sync anymore.
My solution in that case is just to delete the lock file in the folder, and then reconnect the Joplin instances one at a time, waiting the first one's sync to be over, then the second, etc.