Fix WooCommerce Checkout Stuck on Loading (wc-ajax update_order_review 404)

My store’s checkout hangs indefinitely and the console shows a 404 error on the wc-ajax ⇒ update_order_review endpoint. Nothing was added or updated before the problem began, and I’m not even sure what WooCommerce version we are on right now.

I need someone who can:
• Trace why that specific Ajax route returns 404 Not Found instead of the expected JSON.
• Patch or configure whatever is necessary—theme, permalinks, .htaccess, hooks, or core files—so the order-review call succeeds and the checkout page completes without spinning.

Please be ready to work directly on a live site using standard tools such as WP-CLI, browser dev tools, and WooCommerce logs, and to explain the root cause so I can avoid a repeat. Quick turnaround is appreciated; the shop is currently losing sales until this is resolved.