Without explanation, North Korea abruptly canceled its invitation Friday for a U.S. special envoy to visit Pyongyang, bewildering American and South Korean officials who apparently expected the release this week of an American man detained in the communist nation. At the invitation of the North, Amb. Robert King, special envoy on North Korean human rights issues, had planned to travel to the communist nation on Friday (local time) in an effort to free Kenneth Bae, a 45-year-old whose health is reportedly deteriorating.