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When you try to access a mailbox search in Exchange Control Panel (ECP), the Get-MailboxSearch task fails and returns the following error message:   

Microsoft.Exchange.Diagnostics.BlockedDeserializeTypeException: Deserialization of type System.UnitySerializationHolder blocked due to NotInAllow at location MailboxDataStore exception.

Additionally, the mailboxes block the migration to Office365.


 To fix this issue, install the following security update: 

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