hassio-dropbox-sync: All local backup files are "Skipping already existing file" even if Dropbox folder is empty!

After a new install of this great add-on, it seems uploading the backup .tar files to Dropbox does not work for me. The add-on runs fine, it receives the command to upload, but treats all files as ‘already existing’ on Dropbox? Even if the Dropbox app folder is still completely empty.

I use add-on 1.2.0 on hass.io 0.71.0 (installed on a nuc with Ubuntu server 18.04, running in a docker).

I created a new app on Dropbox developers page for this and generated the token, following your instructions on Github.

My config for the add-on is:

{
  "oauth_access_token": "dedacted my access token...",
  "output": ""
}

The log file shows:

Log
starting version 3.2.4
[Info] Files will be uploaded to: /
[Info] Saving OAUTH_ACCESS_TOKEN to /etc/uploader.conf
[Info] Listening for messages via stdin service call...
{"command": "upload"}
[Info] Received message with command upload
[Info] Uploading all .tar files in /backup (skipping those already in Dropbox)
 > Skipping already existing file "/93d293ce.tar"
 > Skipping already existing file "/a98035c8.tar"
 > Skipping already existing file "/b1161847.tar"
 > Skipping already existing file "/b3254871.tar"
 > Skipping already existing file "/cdbf1233.tar"
 > Skipping already existing file "/f969d2cc.tar"
 > Skipping already existing file "/fa844957.tar"
 > Skipping already existing file "/fddc0a72.tar"

Any idea what is going on here?