XML : Amazon MWS Fulfillment Feed - Error Response after processing "one or more items are invalid"

i submit a fulfillment feed and always get the same error (after requesting GetFeedSubmissionResult), my acknowledgement feed works and i can see the updated MerchantOrderId in Sellercentral.

The feed xml is:

  <?xml version="1.0" encoding="UTF-8"?>  <AmazonEnvelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="amzn-envelope.xsd">  <Header>      <DocumentVersion>1.01</DocumentVersion>      <MerchantIdentifier>***</MerchantIdentifier>  </Header>  <MessageType>OrderFulfillment</MessageType>  <Message>      <MessageID>1</MessageID>      <OrderFulfillment>         <AmazonOrderID>302-7735***-***</AmazonOrderID>         <MerchantFulfillmentID>121625</MerchantFulfillmentID>         <FulfillmentDate>2015-12-23T09:03:18+0100</FulfillmentDate>         <FulfillmentData>             <CarrierCode>DHL</CarrierCode>             <ShipperTrackingNumber>1667***</ShipperTrackingNumber>         </FulfillmentData>      <Item>          <AmazonOrderItemCode>52269***</AmazonOrderItemCode>          <MerchantFulfillmentItemID>371064</MerchantFulfillmentItemID>          <Quantity>1</Quantity>      </Item>      </OrderFulfillment>  </Message>  </AmazonEnvelope>    

The Response is:

  <?xml version="1.0" encoding="UTF-8"?>  <AmazonEnvelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="amzn-envelope.xsd">  <Header>      <DocumentVersion>1.02</DocumentVersion>      <MerchantIdentifier>***</MerchantIdentifier>  </Header>  <MessageType>ProcessingReport</MessageType>  <Message>      <MessageID>1</MessageID>      <ProcessingReport>          <DocumentTransactionID>5004***</DocumentTransactionID>          <StatusCode>Complete</StatusCode>          <ProcessingSummary>              <MessagesProcessed>1</MessagesProcessed>              <MessagesSuccessful>0</MessagesSuccessful>              <MessagesWithError>1</MessagesWithError>              <MessagesWithWarning>0</MessagesWithWarning>          </ProcessingSummary>          <Result>              <MessageID>1</MessageID>              <ResultCode>Error</ResultCode>              <ResultMessageCode>25</ResultMessageCode>              <ResultDescription>We are unable to process the XML feed because one or more items are invalid. Please re-submit the feed.</ResultDescription>          </Result>      </ProcessingReport>  </Message>  </AmazonEnvelope>    

I tried with and without the <Item></Item> Part, i tried without <MerchantFulfillmentID>, i tried <MerchantOrderId> instead of <AmazonOrderId>... I also tried <ShippingMethod>Standard</ShippingMethod> but i dont know if "Standard" is a valid value.

So can anyone point me in the right direction - what am i missing here? Is there any neccessary action before the fulfillment feed that i might have missed? Is the <MerchantFulfillmentID> just a value i can choose here or does it have to be set in some feed before?

Thanks!