TypeError thrown when authenticating Dropbox

Hi there,
I’ve just installed this plugin on two sites – my main site and my staging site. I want to back up both of them to my Dropbox account.

First I created a new app in Dropbox, and I entered the Dropbox App key and secret into the API Keys settings on my staging server. Then I configured a backup job there, so it would use Dropbox. I authenticated Dropbox using the “Full Access to Dropbox” field (click the button “Get Dropbox App auth code”).

This worked perfectly, and afterwards I was able to successfully back up my database and files to Dropbox from the staging server.

Then I repeated the same steps on my main site. I used the same Dropbox App key and secret, so it should use the same app in Dropbox.

However, when I edited the backup job, and tried to authenticate Dropbox (clicking the “Get Dropbox App auth code” button), and I pasted in the new auth code into the field for Full Access, I got an error message after I clicked Save:

<strong>TypeError thrown</strong>
Argument 1 passed to GuzzleHttp\Client::send() must be an instance of GuzzleHttp\Message\RequestInterface, instance of GuzzleHttp\Psr7\Request given, called in /wp-content/plugins/backup-wd/vendor/aannnaa7/dropbox-php-sdk-with-cacert/src/Dropbox/Http/Clients/DropboxGuzzleHttpClient.php on line 57

I’ve tried several times but I always get this error. Do you know what it means?