Membership subscription PayPal sandbox error

WordPress 3.5.1
Membership 3.4.3.1

I'm trying to test the subscription payment process using PayPal Express Gateway but ran into an error on the PayPal sandbox website. I logged in using a test sandbox PayPal user and this page: https://www.sandbox.paypal.com/uk/cgi-bin/webscr?cmd=_ ........

displayed:

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

My Membership gateway settings are:

PayPal Site United Kingdom
Paypal Currency GBP
PayPal mode - Test mode (sandbox)

Left the buttons with their default urls.

(I changed the PayPal site to United States - which resulted in the same error)

I'm not sure what I'm doing wrong?

http://205.186.153.30/subscriptions/