I’m following up with Klarna on this issue, but here’s what I’ve got:

Here’s an example HTML form that I copy/pasted from a working forward.html.erb page: https://gist.github.com/edward/115f37ba310a2b6f91bb

Here’s a doc covering the fields that we send Klarna’s HPP: https://hpp-staging-eu.herokuapp.com/api/doc#!/checkout/POST--version-checkout---format-_post_0

Here’s an example form-maker thing that lets you construct a Klarna-valid form and send it to their checkout: https://hpp-staging-eu.herokuapp.com/example/checkout-form/advanced – I’m unable to make it pass an email, phone number, etc. to Klarna.