Hey everyone — ran into something nasty during a recent Exchange Server SE and Windows Server 2025 migration that’s worth sharing.
After running Setup /PrepareSchema for Exchange SE, schema replication between older 2016 DCs and new 2025 DCs completely broke.
Symptoms included:
Event ID 1203 errors
repadmin /showrepl failures
High lsass.exe CPU usage
Duplicate Exchange schema attributes
Turns out this is likely a Microsoft schema duplication bug in Exchange SE. It affects mixed-domain environments with legacy DCs.
I documented the full story, analysis, and the advanced workaround here:
https://expert-advice.org/exchange-server/exchange-server-se-and-active-directory-schema-duplication-bug-a-migration-story/
Would love to hear if anyone else has seen similar behavior or found a more elegant fix.
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