See upstream bug: https://github.com/hashicorp/vagrant/issues/9442
==> default: Checking if box 'debian/contrib-jessie64' is up to date... ==> default: There was a problem while downloading the metadata for your box ==> default: to check for updates. This is not an error, since it is usually due ==> default: to temporary network problems. This is just a warning. The problem ==> default: encountered was: ==> default: ==> default: The requested URL returned error: 404 Not Found ==> default: ==> default: If you want to check for box updates, verify your network connection ==> default: is valid and try again.
This is pretty crappy. :/