Problem
You’ve noticed that the Azure AD Connect server in the on-premise environment is no longer synchronizing with Azure AD and attempting to launch the synchronization engine displays the following error:
AzureADConnect has stopped working
Problem Event Name: CLR20r3
Problem Signature01: AzureADConnect.exe
Attempting to run an AD Sync via PowerShell will indicate that the Synchronization service isn’t running:
Windows PowerShell
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PS C:\Windows\system32> Start-ADSyncSyncCycle -PolicyType Delta
Start-ADSyncSyncCycle : Synchronization Service is not running.
At line:1 char:1
+ Start-ADSyncSyncCycle -PolicyType Delta
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : WriteError: (Microsoft.Ident...ADSyncSyncCycle:StartADSyncSyncCycle) [Start-ADSyncSyncCy
cle], InvalidOperationException
+ FullyQualifiedErrorId : Synchronization Service is not running.,Microsoft.IdentityManagement.PowerShell.Cmdlet.S
tartADSyncSyncCycle
PS C:\Windows\system32>
Solution
One of the reasons why the above errors would be thrown is if the AD Connect server has run out of drive space on the C drive. Increasing the C drive space or cleaning up unneeded files to create space will correct the issue: