PayPal button issue

I’ve added a paypal subscription button from WP Membership but it’s not working properly… I click it, follow the security prompts, then see this error message:

We’re sorry, things don’t appear to be working right now. Please try again later.

I’ve looked at previous posts to see if anything helps and have IPN enabled in my paypal account, but can’t understand what’s missing from the settings that I’ve filled out… Can someone help please?

I noticed you can create a link to advanced paypal settings but don’t understand whether it replaces the existing shortcode or not or what it actually does…

The page I need help with: [log in to see the link]