Checkout Customize page not loading after switching to New Customer Accounts

Hi everyone! Hope you are doing well

After switching Customer Accounts from Legacy → New, the Checkout → Customize page in Shopify Admin no longer loads.

What I’ve confirmed/tried:

  • checkout.liquid is fully removed

  • Toggled Customer Accounts back to Legacy → New

  • Logged in via Incognito / cleared cache

  • Disabled all checkout-related apps

The storefront and checkout work fine outside admin — the issue seems limited to the admin checkout preview after migrating to New Customer Accounts.

Has anyone encountered this before?
Did it require a Shopify support reset, or is there a known workaround?

Thanks!