We have exhaustively investigated the calendar subscriptions issues.

For example on the installed version of office 365 application a URL is able to be subscribed to which is sent via an e-mail to the uset, e.g.,  https://my.sitename.com/smartabase/calendar?userid=24029.

However, it is NOT possible to authenticate the url using the Outlook web application

We built a test case to establish authorization was working correctly from our end and we analyzed the server logs. The basic authentication is not being sent through in the header when specified in the url from the web application. This is reflected in the error message on the client: It looks like this is a private calendar and you don't have permission to view it.

The ONLY possible workaround would be to build a public calendar. This would have no authentication and thus be publicly accessible.

For the installed office 365 version, authentication is required, which is the recommended and safe protocol.