PayPal Sandbox not completing the subscription

Hi, I wonder if you can help me?

I am adding a Subscription membership to a website I run for a charity and have installed the Simple Membership Plugin successfully recreating the pages (I accidentally deleted the original pages). I have created a PayPal account for the charity and this has been verified with PayPal and also created a Sandbox account with the required email addresses.

I have two subscription buttons at the moment (more to follow) but wanted to check that the setup was working ok with the two buttons. The buttons take me to PayPal and have the amount for subscription entered in the payment box, but when I try to login with my ‘buyer’ email address I am presented with a page that says:
“Sorry, an error occurred after you clicked the last link”. I have tried clearing my cache and using Google Chrome in an Incognito window too.

I have tried with a PayPal ‘Advanced Variables’ generated button and that seems to work as does a Braintree one but I would like to have the basic PayPal subscribe because this ties in with my theme better.

The site membership url is: https://hymnsocietygbi.org.uk/membership-join/