Serialization fails when accessing mailbox searches in ECP

Symptoms

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.

Resolution

 To fix this issue, install the following security update: 

• Description of the security update for Microsoft Exchange Server 2019, 2016, and 2013: February 14, 2023 (KB5023038)​​​​​​​