Did you receive an email with the subject "Cirrus Insight Calendar Sync Problem"? If so, that means Calendar Sync wasn't able to access your calendar and you'll need to grant authorization.
To do so, follow these steps:
1. Navigate to https://dashboard.cirrusinsight.com/#/settings/personal/calendar
2. Choose your platform by clicking the radio button beside Office 365.
3. Click the Begin Authorization button.
4. Follow the prompts and accept the necessary permissions.