If you’re watching your directory sync health, or if you have processes that depend on frequent directory sync, you’ll notice the broken sync fairly quickly.
Otherwise, you should receive an email alert after 24 hours to notify you that synchronization is unhealthy.
One of the troubleshooting steps I used was to run Get-ADSyncScheduler in PowerShell on the AAD Connect server itself. In the days leading up to this problem I had opened the AAD Connect configuration to check some things. This pauses the sync schedule, so my thinking was that the schedule had not been re-enabled for some reason. But instead of seeing the expected output, the following error occurred.
PS C:\> Get-ADSyncScheduler Get-ADSyncScheduler : System.InvalidOperationException: Showing a modal dialog box or form when the application is not running in UserInteractive mode is not a valid operation. Specify the ServiceNotification or DefaultDesktopOnly style to display a notification from a service application.
Adding the sync account as an exception to the conditional access policy in Azure AD immediately solved the problem.
The Get-AdSyncScheduler cmdlet now returns the expected results, and the next sync run was successful as well.
PS C:\> Get-ADSyncScheduler AllowedSyncCycleInterval : 00:30:00 CurrentlyEffectiveSyncCycleInterval : 00:30:00 CustomizedSyncCycleInterval : NextSyncCyclePolicyType : Delta NextSyncCycleStartTimeInUTC : 5/21/2018 4:46:16 AM PurgeRunHistoryInterval : 7.00:00:00 SyncCycleEnabled : True MaintenanceEnabled : True StagingModeEnabled : False SchedulerSuspended : False SyncCycleInProgress : True