Membership subscription PayPal sandbox error

WordPress 3.5.1

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: ........


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?