Problem
[GoogleCalendarV3Importer] [Warn] [USER] Appointment: ... failed: Invalid sequence value. The specified sequence number is below the current sequence number of the resource. Re-fetch the resource and use its sequence number on the following request.
Solution
Google recognizes that a calendar event already exists at the destination, and that updates may have happened since the original migration, resulting in the event being out of sync. Therefore, Google rejects the event.
You can remigrate the events by deleting them from the destination, clearing the migration history, and remigrating.
An appointment from the source may include the migrating destination user as an attendee, therefore, the appointment is already in the destination and cannot be migrated.