Membership 2.0 payment issues – 502 error

We’ve been building a site using Membership 2.0 (VideoBiker.co.uk). It’s been a long project & the site is not yet accepting payments. Around nine months ago, when we last checked, the configured Stripe payment gateway was working. Getting ready to launch a payment option, last week we tried to test the Paymentent links. They are returning a 502 error. This was on the live site. We’ve today updated Membership 2.0 to the latest version on our staging site and it too is returning the same 502 error when clicking on a payment link.