ByD Web Service: resending Invoice Request with Attachment fails

Hi experts!

I'm trying to get ByD Invoice Request web service working in a scenario where I want to update the invoice request with an attachment. I'm using the Web Service "ManageCustomerInvoiceRequestIn" and I want to replace whatever is in there with the new invoice request and attachment.

I'm able to do the initial sending and also update without the attachment perfectly. But when I have the attachment-element in the message it fails with a weird error:

            <Item>
               <TypeID>006(//DOC/DS_COMMON/)</TypeID>
               <CategoryCode>SEI.FSI</CategoryCode>
               <SeverityCode>3</SeverityCode>
               <Note>Document 30000000000000000000000000000000 does not exist</Note>
            </Item>

The AttachmentFolder element in the message is configured like this:

            <AttachmentFolder DocumentListCompleteTransmissionIndicator="true" ActionCode="04">
               <Document PropertyListCompleteTransmissionIndicator="true" ActionCode="04">
                  <LinkInternalIndicator>true</LinkInternalIndicator>
                  <VisibleIndicator>true</VisibleIndicator>
                  <CategoryCode>2</CategoryCode>
                  <TypeCode>10001</TypeCode>
                  <MIMECode>application/pdf</MIMECode>
                  <Name>filename.pdf</Name>
                  <AlternativeName>Image</AlternativeName>
                  <Description languageCode="EN">Invoice</Description>
                  <FileContent ActionCode="04">
                     <BinaryObject mimeCode="application/pdf">JVBERi0xLj......

I've read Knut's excellent blog many times and tried different variations with the Actioncodes and LCTIs but I just can't get it working. Any ideas?

Thanks in advance for any help!

Ossi