Use the correct <calendar_name>_shared_by_<user> uri, e.g. work_shared_by_tim, instead of just work (returned by getCalendarById()) which will throw as SabreDAV will try to look for the potentially non-existent or mistake it with the user's own calendars/me/work path rather than the correct calendars/me/work_shared_by_tim path

Note: Contacts export already uses the correct uri

The date is removed from the filename as well as the export zip itself is timestamped, and the underscore no longer has special meaning as a separator