Why does appleid.apple.com/auth/authorize throw an "invalid_client" error?

Hi

https://appleid.apple.com/auth/authorize?client_id=com.adobe.services.adobeid-na1.web

shows:

invalid_request

But https://appleid.apple.com/auth/authorize?client_id=xrqxnpjgps

shows:

invalid_client

I've created a Primary App ID and ticked "Sign In with Apple".

I've created a Service ID and ticked "Sign In with Apple" (identifier is xrqxnpjgps).

When I click "Configure" for the "Sign In with Apple" of the Service ID, it is linked to the Primary App ID.

Why do I get an invalid_client error?

I've contacted the support by mail, and have been redirected here, does someone here have the ability/access/knowledge/will to figure out the cause and then tell me?

Regards