HCI PI - Gather two payloads and transform mapping -> "Cannot produce target element"

Dear Community,

I’d like to add to an given xml-payload some additional information via an external REST-service. The Rest-Serice-call is made by a “request-reply” element. To consolidate the initial payload and the rest-reply I used a join and a gather-Elemen:

The consolidated xml (after the gather-element) looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<multimap:Messages xmlns:multimap="http://sap.com/xi/XI/SplitAndMerge">
  <multimap:Message1>
    <Response xmlns:Response="xs">
    <MetaInfo>
      <Timestamp>2017-08-07T11:27:46.595+0000</Timestamp>
    </MetaInfo>
    <View>
      <Atr1>Example1</Atr1>
      <Atr2>Example2</Atr2>
      <Atr3>Example3</Atr3>
    </View>
  </Response>
</multimap:Message1>
  <multimap:Message2>
    <Customer>
      <Timestamp>2017-08-07T12:07:58Z</Timestamp>
      <EMailAddress>test@test.com</EMailAddress>
    </Customer>
  </multimap:Message2>
</multimap:Messages>

In the last step, I added a mapping, so that the payload is transformed in the needed ODATA-payload structure for hybris Marketing.

Although all necessary information is available, I get the following error message:

com.sap.aii.mappingtool.tf7.IllegalInstanceException: Cannot produce target element /ns0:Messages/ns0:Message1/ImportHeaders/ImportHeader/Contacts/Contact/Timestamp. Queue has not enougth values in context. Target xsd requires a value for this element, but target field mapping does not produce one. Probably the xml-instance is not valid to the source xsd, or the target field mapping does not fulfill the requirement of the target xsd.

Thank you,
best regards,
Peter