It seems that the calendar sharing function of O365 has changed again…

But I got it to work, see the following instructions and screenshots:

1. Visit Outlook Calendar Online App (e.g. https://outlook.live.com/calendar)
2. Go to Settings (in the upper right corner of the window):
   
3. Show all Outlook Settings (or however this is called in English :-) )
4. In the settings window, go to “Calendar” > “Shared Calendars”:
   
   You should now see here two url (html and ics). If not, you first have to choose a calendar (at “Einen Kalender auswählen”) and set the Permissions.
5. Copy the .ics url and put it into your config.js file for your MagicMirror
6. IMPORTANT: Replace the “https” of this url by “webcal” (as it is for the default calendar):
   

This worked perfectly fine for me, but who knows, maybe Microsoft will change this procedure again in a half a year or so… :/