@Valentin same thing hit us out of nowhere a while back with no code changes. A few things worth trying:
clear the CLI cache and restart:
shopify app dev --reset
if that doesn’t help, try opening the preview in a fresh incognito window — sometimes the dev console caches a broken state.
also worth checking if the issue is specific to customer-account-ui or if other extension previews load fine. if everything else works its likely something on shopify’s preview side for that extension type rather than your setup.
since you’re seeing it across multiple apps and stores it does sound like a platform side thing that started this morning, might be worth flagging to Shopify support with your CLI version so they can check if others are hitting the same thing.