"You cannot pay the seller through un-encrypted buttons."- error msg Membership plugin

Multisite 3.1.1, Membership plugin.

Hi, I created the registration page for the Membership plugin and inserted the [subscriptionform] code on the page.
When a user clicks on one of the buttons to subscribe he gets this error message:
"Return to Merchant
Error Message
The seller accepts encrypted website payments only. You cannot pay the seller through un-encrypted buttons. Please contact your seller for more details."

The membership button is the default one provided
Membership> Edit Gateways>Paypal>Button> the default provided is https://www.paypal.com/en_US/i/btn/btn_subscribe_LG.gif

How do I fix this?