For the request:
{"path":"/Get Started with Dropbox.pdf","settings":{"requested_visibility":"public","expires":"2016-03-04T14:27:00Z"}}
I got the response always, and it passed if I removed expires.
{"error_summary": "settings_error/not_authorized/", "error": {".tag": "settings_error", "settings_error": {".tag": "not_authorized"}}}
Known issue?