Well, even if it has been generally available for months, seems Windows Server 2025 is in fact still not yet ready for production use.
After multiple core services issues, from DHCP services to authentication with Windows Hello, we now have a new issue which is impacting another system, in this case Exchange server on-premises.
As you know, when deploying Exchange server on-premises – including some cumulative updates, there is a need for Active Directory Schema Extension updates.
Well, it has been reported and confirmed by Microsoft that if you run a Windows Server 2025 Domain Controller hosting the schema master Flexible Single Master Operation (FSMO), you will face replication issues after installing the latest Exchange Server 2019 Cumulative Update (CU15) or Exchange Subscription Edition due to attribute duplication issue.
This is being reported with the following application event log events:
- Error 8418: The replication operation failed because of a schema mismatch between the servers involved.
- Warning 1203 (NTDS Replication): The local domain controller could not replicate the following object from the source domain controller at the following network address because of an Active Directory schema mismatch.
This has been acknowledged by Microsoft with the support KB article KB 5065426 September 9, 2025—KB5065426 (OS Build 26100.6584) – Microsoft Support.
To avoid any issue when deploying Exchanger Server 2019 CU or Exchange Subscription Edition, ensure the Schema Master FSMO role is not assigned to a Windows Server 2025 domain controller.
If it is already too late, you need to open a support request (which will be free as this is a product bug).
Official announcement Active Directory schema extension issue if you use a Windows Server 2025 schema master role | Microsoft Community Hub