The error occurs when migrating to a G Suite tenant. The Google Calendar API can update a calendar event only once.
When MigrationWiz attempts to create some calendar events, Google recognizes that the event already exists at destination, and that updates may have happened since the original migration resulting in the event being out of sync and rejected by G Suite. The migration fails.
You can resolve the synchronization issue by deleting the offending items on the Destination, and then restarting the migration.