Why is the Shopify GraphiQL App showing a 'page doesn't exist' error?

Trying to use the Shopify GraphiQL App results in an “The page you were looking for doesn’t exist.” error. The App was installed in multiple projects ages ago.

It still can be installed (from the Shopify Graph document page), toogling between fullscreen app view and admin-Page and then opening the configuration with a cookie-warning. ??

This has been happening to me too on Firefox for the last few days. Curious if there’s any update from Shopify.