For some kind of reason, this is a syntax that works:

SELECT Client.Name,SUBSTRING(COALESCE((SELECT ', '+RTRIM(Invoice.ID)
 FROM ClientInvoice (NOLOCK)
 INNER JOIN Invoice ON Client.NoInvoice=Invoice.Numero
 WHERE ClientInvoice.NoClient=Client.Numero FOR XML PATH (''),TYPE).value('.','varchar(max)'),''),3,1000) AS InvoiceID
 FROM Client

I just do not understand how the other was working and I had to adjust it tonight to fix this issue.