Active Directory schema extension issue if you use a Windows Server 2025 schema master role

The Workaround

While Microsoft hasn’t yet released an official hotfix or KB article as of October 2025, here’s what worked in this environment:

1- Isolate the schema master role (FSMO) on a Windows Server 2025 DC.
2- Export the schema using ldifde -f schema.ldf -d "CN=Schema,CN=Configuration,DC=...".
3- Manually locate duplicate attributes (often Exchange-related, such as msExchMailboxGuid or msExchAttributeSet).
4- Compare attribute OIDs and GUIDs—retain the newer SE versions.
5- Clean orphaned or duplicated entries via adsiedit.msc or ldifde imports (extreme caution required).
6- Force replication again using: repadmin /syncall /A /e /P
7- Once replication stabilizes, demote the older DCs and complete the migration.