WHMCS Integration problem

Hi,

I have configured everything and it works fine until the order step 4. The checkout button became to have weird characters(Checkout%26raquo) and Paypal Express Checkout lead to an error :

"Sorry this doesn't look like a WHMCS site at http://client.pagecoco.com/ 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."

Please help on this issue.
Best,
Antika