Setup encountered a problem while validating the state of Active Directory.
If you are running load-balanced Client Access servers in a site then you should configure the load balancer to remove the server from the pool of hosts, and allow any existing connections to close, before you install the cumulative update.
Note: The AD preparation tasks are not required to be run separately to the upgrade of Exchange, unless in circumstances where you need to separate the tasks to different teams with different permissions, or if you have a multi-domain forest and want to control the AD changes.
Before applying the schema update follow the steps provided by Michael B Smith to retrieve the existing Exchange schema version, so that you can compare it before and after the AD preparation steps have been completed to verify that the schema update was applied.
In this article I will demonstrate the step by step process for installing cumulative updates and service packs for Exchange Server 2013.
The steps for installing cumulative updates and service packs on Exchange 2013 are: If you have multi-role CAS/MBX servers installed then setup updates the roles in the correct order anyway, and you should simply start with the internet-facing servers.
In other words, you should plan to update all DAG members within a short period of time, and not allow them to run at different versions for days, weeks or months.