If you tried to change your plan or add an add-on since 10 August, the payment page did not open. Pressing “Continue to secure checkout” sent you to the sign-in screen instead — even though you were already signed in. Signing in again just brought you back to the same place. “Manage existing billing” was affected in the same way.
What went wrong. When we moved sign-in to its own secure service on 10 August, the way your session is stored changed so that it can no longer be read by anything running in the page. That is deliberate and it is the safer arrangement. The checkout page, though, was still deciding whether you were signed in by looking for the old details — found nothing, and concluded you were a stranger. It never actually asked. Your account, your plan and your payment details were never at risk, and nothing was ever charged incorrectly; the page simply refused to go any further.
What happens now. The checkout page asks our systems whether you are signed in rather than guessing, so it opens straight away. If you genuinely are signed out it takes you to sign in and brings you back to what you were buying. New customers coming from the pricing page still go through sign-up with their chosen plan intact.
Nothing is needed from you. If you gave up on an upgrade or an add-on in the last two days, it will work now.