IWBEP/CX_MGW_BUSI_EXCEPTION on deep insert of SalesOrders via Rest client

Hi all,

I am performing deep insert operations on sap gateway demo services https://sapes1.sapdevcenter.com/sap/opu/odata/sap/ZCD204_EPM_DEMO_SRV/SalesOrders
via Advance rest client.
X-CSRF-TOKEN is in the headers and  payload has

<?xml version="1.0" encoding="UTF-8"?>

<atom:entry xmlns:atom="http://www.w3.org/2005/Atom" xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">

   <atom:content type="application/xml">

    <m:properties> 

<d:SalesOrderID></d:SalesOrderID>

<d:Tax>0.00</d:Tax>

<d:Currency>EUR</d:Currency>   

<d:CustomerName>SAROSH</d:CustomerName>

<d:BusinessPartnerID>0100000079</d:BusinessPartnerID>

</m:properties>

   </atom:content>

<atom:link

rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/SalesOrderItems"

type="application/atom+xml;type=feed" >

<m:inline>

<atom:feed>

<atom:entry>

<atom:content type="application/xml">

<m:properties>

<d:QuantityUnit>EA</d:QuantityUnit>

<d:Tax>87.4</d:Tax>

<d:ProductName>EPM DG: SO ID 0500000097 Item 0000000010</d:ProductName>

<d:ProductID>HT-1030</d:ProductID>

<d:Position></d:Position>

<d:SalesOrderID></d:SalesOrderID>

<d:Currency></d:Currency>

<d:Quantity>2.0</d:Quantity>

<d:DeliveryDate>2014-08-14T02:00:00.000</d:DeliveryDate>

</m:properties>

</atom:content>

</atom:entry>

</atom:feed>

</m:inline>

</atom:link>

</atom:entry>

while POST method I am getting following output

<errorxmlns="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">

</error>

Any help would be highly appreciated

Best Regards
Sarosh Bilal