Today i had the chance to encounter for the first time this issue – Azure AD is not synchronizing users from on-premises Active Directory to Azure AD anymore.
Having a quick look on the on-premises, Azure AD Sync server, i have noticed the following event being logged in Windows Logs -> Application, continously:
Log Name: Application
Source: Directory Synchronization
Event ID: 904
Scheduler::SchedulerThreadMain : Maintenance or Sync cycle not being run because mutex acquisition failed. Will retry in 10 seconds.
Found another user logged onto the server, with a disconnected session, but having the Synchronization Service Manager Console still open in his/her session. Logged off the user completely, then the above events stopped getting logged and the Sync Cycles started running.