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: (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=;;***
[2012-12-31 09:10:06][M] 03 - sending request - response=        <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">
                <title>Toegang geweigerd.</title>
                <link rel="stylesheet" href="" type="text/css" />

            <body id="error-page">

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



[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:
Jan & Kristof