Return to Merchant — PayPal link error

Hi, suddenly started getting this (below) today after years with no problems.

Buttons are on this page: http://www.fapsa.org.uk/membership-options/

S2Member Pro with all plugins up to date.

I’ve seen this happen whenever a new email is added to your PayPal account. Did you change it recently? If so, have you completed the full verification required for that new email address to be used?