In order to run processes like Calendar Sync, Email Sync, or Contact Sync, Cirrus Insight needs access to your Google account. Occasionally, there will be a problem with the authorization and you'll need to grant access again.
To provide some background about this type of situation, both Salesforce and Google have default behaviors in certain situations that notify event attendees of a declined/cancelled event if the event is deleted. Now, the only time Calendar Sync should be causing an event to be deleted is when the synced event is first deleted from the other calendar.
Your organization has designated a setting that blocks events from being synced if all the attendees’ email addresses share the same domain as you. This setting is controlled by your organization’s Cirrus Insight admin.