We noted that DirSync wasn’t working for one of our customers, new users weren’t being provisioned in the cloud and there were errors in the event log about FIMS and DirSync.
The issue occurred because the cloud account used to access Office 365 had its password changed. The solution is to re-run the the Windows Azure Active Directory Synchronisation tool and enter the new credentials.
If you get this error “Access to registry key ‘HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSOLCoExistance’ is denied.” while running the tool after you enter the Azure AD cred’s, you just need to launch the tool as administrator from the shortcut.
Errors in the event log
Event 0, Directory Synchronization
Event 652, Directory Synchronization
Event 6900 FIMSynchronizationService
Event 6329, FIMSynchronizationServer