To-Do Preview is being made availableto customers "off-by-default". This means admins must explicitly opt-in to use of the preview apps, using the toggle in the "To-Do Preview" section in the "Services & Add-ins" page of the O365 admin settings. Until tenant admins flip this switch for their organizations, users will receive an error explaining this requirement during login and will be logged out by the app.
Please note we are aware of an issue that is currently causing the "To-Do Preview" settings pane to be hidden for select customers within the announced release. We are working to resolve this as soon as possible.