Query list filter by AssignedTo

I'm trying to create a workflow which should list all open tasks from a person.

Somehow I cannot filter by the username. It always shows me an empty result, eventhough when I delete the <where> statement, I can see the the AssignedTo value, which I'm filtering for.

I have seen this post, but sadly it doesn't work for me:

Query List action by assigned to

Thanks for your help :)

<Query>
  <Lists>
    <List ID="{675F51CD-8654-4CB4-85E1-BE4DAAB73107}" />
  </Lists>
  <ViewFields>
    <FieldRef Name="AssignedTo" />
  </ViewFields>
  <Where>
    <Eq>
      <FieldRef Name="AssignedTo" LookupId="True" />
      <Value Type="User">: i:0#.w|DomainDomainuser</Value>
    </Eq>
  </Where>
</Query>