Troubleshooting Azure AD Connect Sync Diagnosing Kerberos and Replication Errors

Summary: 

When Azure AD Connect sync breaks, the error in the sync engine is often a symptom, not the cause. This post walks through how to diagnose Azure AD Connect sync errors rooted in Kerberos authentication and Active Directory replication, and the order in which to check them. 

A failed directory synchronization process rarely starts in Microsoft Entra Connect (Azure AD Connect) itself. More often, the connector server is simply reporting that something underneath it—time, authentication, or replication—is already broken on-premises. Treating the sync error as the starting point usually leads to a long detour. The faster path is to confirm the foundation first, then work up to the sync engine. 

The most common culprit is Kerberos authentication between the connector server and the domain controllers. Kerberos is intolerant of clock drift: if the connector server and the DCs differ by more than five minutes, ticket validation fails, and the sync service account can no longer authenticate. Duplicate or missing service principal names (SPNs) and stale cached credentials produce the same result. A quick check with w32tm and a review of the sync account’s SPNs resolves a surprising share of Azure AD sync issues and unexpected synchronization failures. 

The second layer is Active Directory replication. If the domain controllers are not replicating cleanly, the source data feeding the sync is inconsistent before the connector ever runs. AD replication errors in the Directory Service event log are a clear signal to pause and fix Active Directory health before touching Azure AD Connect. Synchronizing from an unhealthy directory only pushes inconsistent objects into the cloud and creates additional Azure AD Connect sync errors. 

A practical troubleshooting order 

• Confirm replication of health across all domain controllers

Use repadmin /replsummary and dcdiag to verify domain controller replication and overall Active Directory health before anything else. Resolving AD replication errors early prevents downstream synchronization problems. 

• Verify time synchronization on the connector server and DCs

Check time synchronization with w32tm to rule out Kerberos clock skew, one of the most common causes of Kerberos authentication failures and Azure AD Connect sync interruptions. 

• Validatethe sync service account 

Confirm the Azure AD Connect service account is not locked or expired and check for missing or duplicate SPNs. Authentication issues at this level often result in recurring Azure AD Connect sync errors. 

• Review connector and staging status in the Synchronization Service Manager

Review connector health and staging status to isolate where objects are failing within the directory synchronization process. This helps identify whether the issue is related to connectivity, permissions, or synchronization rules. 

• Only after the foundation is healthy, force a delta sync and confirm objects flow through cleanly

Once Active Directory replication, Kerberos authentication, and account health checks are complete, perform a delta sync and verify that objects are successfully synchronized between on-premises Active Directory and Microsoft Entra ID. 

The lesson from this work is simple but easy to forget under pressure: fix the on-premises foundation—Active Directory replication and Kerberos authentication—before chasing the sync engine. Most Azure AD Connect troubleshooting efforts become significantly easier when replication, authentication, and time synchronization are verified first. The sync errors are downstream symptoms, and resolving the underlying issues clears most of them at once. Working in that order turns a multi-hour investigation into a focused, repeatable Azure AD Connect troubleshooting checklist.