When running an Office 365 with Active Directory synchronisation from your on-premise domain, you may encounter the issue where passwords for one, some or all accounts do not sync anymore. This can particularly happen if changes were made to the configuration of the AD sync (such as change in OUs, change of rules, etc.).

If you're using Azure Active Directory Sync tool (old method) for synchronisation, on the server running AD sync, start Powershell, then run these:

Import-Module DirSyncSet-FullPasswordSyncRestart-Service FIMSynchronizationService -Force

Run the following Powershell script on the server running Azure AD Sync Service or Azure AD Connect:

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