A customer had several of these errors being reported in Azure AD Connect. They were all users which had been converted to cloud-only. Several solutions are on Microsoft and related sites, none worked except this on the domain controller which has AAC:
- Open administrative Powershell
- To stop synchronization:
set-adsyncscheduler -synccycleenabled $false - In the AAC Synchronization Service Manager, click Connectors
- Right-click on the AD connector, click delete, choose “Deleted connector space only”, click OK
- Do the same for the Azure connector
- To restart synchronization:
set-adsyncscheduler -synccycleenabled $true - In services.msc, restart “Microsoft Azure AD Sync”
- Reinitialize sync:
Start-ADSyncSyncCycle -PolicyType Initial