TestFlight broken on all my apps: invites never arrive + "The requested app is not available or doesn't exist"

Hi everyone,

I'm stuck with a TestFlight issue affecting ALL apps on my developer account:

1. New internal testers show "Invited", but never receive the invitation email. "Resend Invite" does nothing.
2. Existing testers get notified about new builds, but installing fails with: "Could not install [App]. The requested app is not available or doesn't exist."

I've contacted Apple Support but no answer yet.

Has anyone had this recently? How did you fix it?

Thanks!