Claim not granted when activating add-on

Hi! I am transferring a client website to a new Domain. When I install the UDP on the new site, then click the Add-ons Tab, then enter my account details to login to your site, then click the “Activate it on this site” link, all I get is a pop-up message, saying – Claim not granted – perhaps you used this purchase elsewhere.

We have created many new client websites by using another client site as a template and transferring it via UDP. It has always worked without any issues until now. Any ideas?