Some more information regarding this. The asmx webservice is expecting this in the request:
<mx:PreagreementStatement xsi:nil="true"/>
Does Mendix cater for this nillable attribute when it generates the webservice request in the mapping, and if so how does one configure this in the mendix domain to xml mapping?
This should work, and an xsi:nil should show up in the webservice call. Would you mind filing a bugreport in the support portal?