Stripe with custom child theme of Twenty-One

[niels 1]

When I create a child theme, consisting only of theme.yaml and no other files, the Stripe gateway settings complain that Required Template Changes are Not Found.

My theme.yaml contains only this for now:

name: "My Website"
author: "Me"
config:
  parent: twenty-one

Of course I want to make more changes to my theme than just create a theme.yaml, but it's unclear why Stripe breaks at this point already or how I should fix it.

It is my understanding that WHMCS will look at the parent theme for all files not present in the child theme. And when I play with the theme itself, it does indeed work that way. But the Stripe settings don't seem to pick up on this?

EDIT: the cart theme is standard_cart, unmodified.

Edited January 8 by niels