Unable to move mailbox to Microsoft 365 - User is already being moved

When migrating a mailbox from on premises Exchange Server to Microsoft / Office 365, it failed with below error message.

Error: MigrationPermanentException: The onboarding move could not be created because user '<username>' is already being moved. --> The onboarding move could not be created because user '<username>' is already being moved.

To resolve the issue, check on-premises Exchange server for any existing move request for the user. If yes, remove the move request.

If there is no move request in on-premises Exchange server, check the following attributes for the user in on-premises AD and clear them.

msExchMailboxMoveRemoteHostName

msExchMailboxMoveSourceMDBLink

msExchMailboxMoveSourceArchiveMDBLink

msExchMailboxMoveTargetArchiveMDBLink

Then go back to Microsoft 365 Exchange Admin Center (EAC) and resume the move request.

For more information see MS KB.