This error is coming up because the PayPal profile is set to only accept payments from encrypted buttons.
To turn off this option:
- Log in to your account at https://www.paypal.com/ by entering your email address and password in the "Member Login" box.
- Click the Profile subtab.
- Click on the Website Payment Preferences link in the "Selling Preferences" column.
- In the section titled "Encrypted Website Payments" select Off.
- Click Save.