Membership 2 pro issue with Paypal sandbox

Hi There

I have set up membership2 pro on my dev site and am trying to test with Paypal sandbox. When I click to pay for the membership it links through to paypal but as soon as I click to login or pay via debit or credit card I get the message -

The link you have used to enter the PayPal system is invalid. Please review the link and try again.

Can you advise what I need to do?

Many Thanks