For Exchange 2013 organizations creating new or managing an existing hybrid configuration with the HCW, the following HCW error message indicates you are experiencing the issue this update addresses:
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: Microsoft.Exchange.Data.Directory.DirectoryBackendType
The fix is not required if you already have a hybrid deployment and do not have a need to run the HCW again.
If you currently have an Exchange 2013-based hybrid deployment configured, you will not notice any issues unless you rerun the HCW as part of updating or managing your existing hybrid features. Unless you need to reconfigure your hybrid deployment, you can simply wait for the next update of Exchange Server 2013 (Cumulative Update 6) to correct this issue with the HCW.
Microsoft also provides an FAQ here which should cover any concerns or confusion people may have about this issue.
If for some reason you are unable to acquire the fix from Microsoft Support you could use the steps documented by Steve Goodman here to manually set up a hybrid configuration. However please pay attention to Steve’s warnings and caveats about this approach.