pass on Transformer generated output to Channel

[hl7.sentinel]

Hi there:

Mirth 1.5 with latest patch

I am trying to transform ORU messages to DTF (flat files). For this I set up 2 channels:

OUT_1_ORU: connector type: File Reader
destination: Destination 1 -> File Writer
destination: Destination 2 -> Channel Writer to OUT_2_DTF

OUT_2_DTF: connector type: Channel Reader (incoming data set to XML)
destination: Destination 1 -> File Writer

Destination 2 contains the Transformer (all steps of type Message Builder). I realized that the output of the transformation is XML. So I intended to transfer XML to plain HL7 vertical bar encoded files with channel OUT_2_DTF.

This gives me following error on OUT_1_ORU Destination 2:
FAILURE: Error routing message
RoutingException
Failed to transform message before applying the filter. Failed to route event via endpoint:
...

and following error on OUT_2_DTF Destination 1:
ERROR-301: Transformer conversion error
ERROR MESSAGE: Error adapting message
org.xml.sax.SAXParseException: Content is not allowed in prolog.
...

Do I have an error in reasoning?
It would be great if someone could let me know where to start. (Probably I misunderstood the use of Channel Writer - Channel Reader?)

Thanks a lot in advance.
Best wishes,
Joe