Hi,
The server may not have CURL enabled, or may have a firewall to access the download URI or something similar (security plugin etc..). I checked the server database, I see the request from that purchase code, so that went through all right from our side. One other thing could be, that the server has an file upload limit via a php.ini configuration, and may not allow the access to files above 2MB. I suggest checking that first, as most plugins are way beyond 2MB of file size, and you can bump into similar issues in the future.
Try manual updates via uploading the plugin file instead, it should work all right. If that does not work either, then you should ask your server administrator what could be wrong.