Hi,
Calendar module uses node-ical. There is one issue on Google calendar and Microsoft ism.
See more: jens-maus/node-ical#169

They have fixed this to node-ical 0.15.0 version jens-maus/node-ical#170 & https://github.com/jens-maus/node-ical/commits/0.15.0

Maybe MM can update this also so more ics calendars are working again :)