The requesting account does not have permission to serialize tokens. Error: ErrorTokenSerializationDenied
This error occurs if the caller tries to perform a token serialization request but does not have MS-Exch-EPI-TokenSerialization rights on the Client Access server they are calling.
When migrating to Office 365, this error is usually transient and surrounded by other connectivity errors. Those errors are due to Service degradation in Microsoft Office 365, and will usually fix themselves one service is restored.
Resubmit your migration.