Issue when calling SP web service to populate the people picker field

My nintex workflow calls sharepoint web service to populate a people picker field. I always got error:

'0x81020014 One or more field types are not installed properly. Go to the list settings page to delete these fields.'

I tried 'FName LName', 'domainfname_lname' and 'userid;#fname lname', none of them worked. I got the same error.

What name format should I use to populate this field? 

Here is my SOAP code to update the people picker column.  Thanks

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:m="http://schemas.microsoft.com/sharepoint/soap/">
  <soap:Header>
  </soap:Header>
  <soap:Body>
        <UpdateListItems xmlns="http://schemas.microsoft.com/sharepoint/soap/">
           <listName>Project</listName>
             <updates>
                  <Batch OnError="Continue">
                  <Method ID="1" Cmd="Update">
                      <Field Name="ID">1</Field>
                       <Field Name="ItemProperty:Project_x0020_Lead_x002f_Project">{WorkflowVariable:vName2}</Field>
                       <Field Name="ItemProperty:Project_x0020_Sponsor_x002f_Exec">{WorkflowVariable:vName3}</Field>
                   </Method>
                    </Batch>             
                 </updates>
         </UpdateListItems>
  </soap:Body>
</soap:Envelope>