User Sync plugin not working on Wordpress installations

Hi guys,
We purchased the User Sync plugin last week and tried to set it up between the following two sites:
http://www.hunting.be
http://tableau.hunting.be (running a ClassiPress theme for affiliates)

- When performing the sync in debug mode, I get the Access Denied message in the logs.
- The backend of the subsite is reachable (not blocked on htaccess or whatever) and is enabled in the Theme
- When i access the admin-ajax.php file directly, it returns me a 0.

The error message:

[2012-12-31 09:10:06][M] 02 - sending request - url=http://tableau.hunting.be/wp-admin/admin-ajax.php?action=user_sync_api;;***
[2012-12-31 09:10:06][M] 03 - sending request - response=        <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
        <html>
            <head>
                <title>Toegang geweigerd.</title>
                <link rel="stylesheet" href="http://tableau.hunting.be/wp-admin/css/install.css" type="text/css" />
            </head>

            <body id="error-page">

                <p>Toegang geweigerd. Uw site beheerder heeft uw toegang geblokkeerd tot het WordPress back-office.</p>

            </body>

        </html>

;;***
[2012-12-31 09:10:06][M] 06 - checking key false;;***

Can anyone be of any help to us? The sync is just not working :slight_frown:
Thanks!
Jan & Kristof