Verification Loop

I bought the "PrompSmart Pro - Teleprompter" app about a year ago, and now it won't update (on iPad via App Store).

I'm signed in, my billing info is correct, but it only says "Verification required. Tap continue and sign in to view billing information." Then the button says OK, which I press, and nothing happens. Nothing changes. Everything else I have, paid & free, is updating seamlessly.

I have gone into my account via a browser and updated and confirmed all billing details. I've reset my password.

As per another thread's suggestion with the same lack of success there, I've logged out and closed the App Store, restarted the device, logged back in, and the problem persists.

What's the problem?! Please help.