The iOS app version 1.6 has a bug that crashes at account registration (it still works if one register an account from the web interface then logs in to that account). We already submitted a bug fixed version 1.6.1 to AppStore for review 4 days ago. It should appear around Nov 11.
Unfortunately, the AppStore does not have capability to roll back to previous version when the latest version is approved by AppStore review staff. To prevent more user installing this faulty version of iOS app we have temporarily delisted the iOS app from AppStore until Nov. 11 when version 1.6.1 is expected to the approved.