AML throws Aras.Server.Core.AmbiguousCriteriaException

Thanks for your response, Zahar. Our SQL instance is case-insensitive, so the second where works as it should, if I run the <item> inside the datasource on its own, it returns:

<Result>
      <Item type="Property" typeId="26D7CD4E033242148E2724D3D054B4D3" id="07912B9CE3374ABFAACF1B19B3F6F718">
        <id keyed_name="locked_by_id" type="Property">07912B9CE3374ABFAACF1B19B3F6F718</id>
      </Item>
    </Result>

I'm aware as well about the first where, but currently we only have one property with that name. I'm trying to achieve:

UPDATE innovator.[PROPERTY]
SET DATA_SOURCE = (
	SELECT [PROPERTY].id id
	FROM innovator.[PROPERTY]
	WHERE keyed_name = 'locked_by_id' AND [Property].source_id in (select IT.ID from innovator.ITEMTYPE IT where IT.name = 'Document') )
WHERE [Property].keyed_name = 'era_locked_by'

But I get the feeling that AML can't cope with nested where statements. I'm missing something but I can't put my finger on it.