It seems no http 401 error happens when using RESTful API without authentication.
curl
http://XXX/latest_builds?parent_configuration_id=1&recursive=true ( no -u option)
=> 401 Error happens before 3.1.64
=> Empty XML returns after 3.1.64, including new QB 4.
Most libraries (including httplib2) authenticate just after when 401 error happens.
This bug makes applications using RESTful APIs get empty data even if they pass ID and password to library.