It looks like Google missed adding a redirect from the root google.com domain to the new address on the https secure version of the URL.
i.e.
Old URL https://google.com/calendar/ does *not* automatically forward to new https://calendar.google.com/calendar/
but
Old http://google.com/calendar/ *does* automaticallly forward to new https://calendar.google.com/calendar/
However ...
www.google.com/calendar redirects properly from both http & https
Hope that's helpful