I came across this error when running the Hybrid Configuration Wizard on my Exchange Server 2013 SP1 server. Detailed error :
Subtask CheckPrereqs execution failed: Check Tenant Prerequisites
Deserialization fails due to one SerializationException: Microsoft.Exchange.Compliance.Serialization.Formatters.BlockedTypeException: The type to be (de)serialized is not allowed:
Towards the end of the error, it asked to view the Hybrid Configuration log for more information. You can find the log in the following location of your Exchange 2013 server : C:\Program Files\Microsoft\Exchange Server\V15\Logging\Update-HybridConfiguration.
Upon searching on the issue, I came across a Microsoft article that says this issue occurs due to a recent change in Microsoft’s Exchange Online environment that prevents the Exchange 2013 HCW to run correctly. The issue can be resolved by installing the latest cumulative update. In my environment I had to download the CU6 update to resolve this issue.
Reference : https://support.microsoft.com/en-us/help/2988229/-subtask-checkprereqs-execution-failed-error-in-hybrid-configuration-wizard-for-exchange-server-2013