Update - Resolved!
If you login via CF but you have no teams configured with CF authentication, then this is the error that you get. DOH!
It appears that the redirect is being made to /sky/callback which doesn’t exist instead of the configured /sky/issuer/callback:
After logging into CF from concourse:
This site can’t be reached
The webpage at https://concourse-test.gaig.com/sky/callback?code=zy3tm4jz5b5a2ckbmns6reevk&state=eyJyZWRpcmVjdF91cmkiOiIvIiwiZW50cm9weSI6ImI2OTE3M2Q5NzE0MGNhNDJmNzFjNjY5YjA2NDc0ZTMxN2IxN2ViMThhNTBiNmQ5NDE3OWVkMmIwYzg5MDJmMTQifQ%3D%3D might be temporarily down or it may have moved permanently to a new web address.
scope: cloud_controller.read openid
authorized_grant_types: refresh_token authorization_code