Error : MigrateFolderContent :Unable to cast object of type 'Microsoft.Exchange.WebServices.Data.EmailMessage' to type 'Microsoft.Exchange.WebServices.Data.Appointment'.

This error can occur if there are corrupted attachments within a calendar entry.

It can also occur when email items are found within a calendar folder.

To resolve:

You can look for both issues by viewing the calendar in list view within outlook and looking for a mailbox icon in the first column (most will be the calendar icon) and the paperclip item in the attachments column.