Cannot complete checkout | o.ownerWindow.postMessage is not a function

[ablears]

I have 4.6.7 installed on a WP network child site. Submitting checkout leads to a "Thank you for your order, please click the button below to pay with Razorpay." page. There, clicking 'Pay Now' is unresponsive, and I see the following in the dev console.

Uncaught TypeError: o.ownerWindow.postMessage is not a function
value https://checkout.razorpay.com/v1/checkout.js:1
38111 https://checkout.razorpay.com/v1/checkout.js:1
checkout.js:1:74804

I guess because of this error the RazorPay JS object is not available, which causes the local plugin script to fail.

Uncaught ReferenceError: Razorpay is not defined
https://zzz.com/wp-content/plugins/woo-razorpay/script.js?ver=6.5.4:38
https://zzz.com/wp-content/plugins/woo-razorpay/script.js?ver=6.5.4:63
script.js:38:28

I've tested on Libreoffice on macOS, Chromium on RPI, and Chrome (urgh) on Windows (shudder), and all exhibit this issue.

Adding this script directly through the dev console also shows the same error.

var script = document.createElement('script'); script.src = 'https://checkout.razorpay.com/v1/checkout.js'; document.head.appendChild(script);

GET https://checkout.razorpay.com/v1/checkout.js
[HTTP/1.1 200 OK 212ms]
Uncaught TypeError: o.ownerWindow.postMessage is not a function