WHMCS integration plugin - downloads.php link not working

Hi

I purchased WHMCS integration plugin, all is working fine, except the downloads.php page from WHMCS gives an error:
Sorry this doesn't look like a WHMCS site at [https://www.tuxsp.pt/] Make sure your WHMCS Integration settings are pointing to the correct URL and that the WHMCS site is set for the Portal template in Setup | General.

It's weird since everything is configured properly, and to prove that is that all other link work. Please test:

http://www.tuxsp.pt/tux/area-de-clientes/

link not working: http://www.tuxsp.pt/tux/area-de-clientes/?whmcsportal%5Bpage%5D=whmcss%3A%2F%2Fwww.tuxsp.pt%2Fdownloads.php