whmcs order Sorry this doesn't look like a WHMCS site at [https://clients.beltex.nl/] Make sure

Hi all, i have an other thingy with the whmcs plugin:

http://www.snelgoedkoop.nl/wordpress-webdesign/account/beheer/cart.php/?a=add&pid=26

shows the prder form

but after filling in a bogus domain name and press continue i get

Sorry this doesn't look like a WHMCS site at [https://clients.beltex.nl/] 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.
Sorry this doesn't look like a WHMCS site at [https://clients.beltex.nl/] 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.

But thats ok https://clients.beltex.nl/clientarea.php

If i go to the cart again http://www.snelgoedkoop.nl/wordpress-webdesign/account/beheer/cart.php/?a=add&pid=26

and then select the same product via choose product i am more succesfull and even the old domain which gave me the error is in the cart