WSO2 ESB Content is not allowed in prolog exception in a OutSecuence - web-services

Question

I am developing a proxy service with a soap webservice that performs soap to rest conversion, the message is sent to a servlet that response with a string in flat format (not xml), just a secuence of characters like
OIUW|ECHNOWE|RFHQWIUE|FBPQW|EFHAO|IEFH
I am invoking with SOAP UI and I get this response fine, now I would like to receive it in "SOAP format", wrapping the message into a soap:body, I've tried with a XSLT and with a PayloadFactory Mediator, but as soon as I use any of them (even doing nothing) I get a
[2014-07-31 09:30:41,847] ERROR - RelayUtils Error while building Passthrough stream
org.apache.axiom.om.OMException: javax.xml.stream.XMLStreamException: ParseError at [row,col]:[1,1]
Message: Content is not allowed in prolog.
What do I do wrong ? How can I achieve a message transformation without this exception?
Thank you!
UPDATE: My proxy as requested by Ratha
<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse"
name="SCL3"
transports="http"
startOnLoad="true"
trace="disable">
<description/>
<target>
<inSequence>
<log level="custom">
<property name="MyTrace" value="--- REQUEST ---"/>
</log>
<log level="full"/>
<property name="REST_URL_POSTFIX"
value="x4?msg=x4|0003|0000000021|0|0|0400002081020224849"
scope="axis2"
type="STRING"/>
<property name="HTTP_METHOD" value="GET" scope="axis2" type="STRING"/>
<property name="SOAPAction" scope="default" action="remove"/>
<header name="Action" scope="default" action="remove"/>
<send>
<endpoint>
<address uri="http://localhost:8087/X4" format="pox"/>
</endpoint>
</send>
</inSequence>
<outSequence>
<log level="custom">
<property name="MyTrace" value="--- RESPONSE ---"/>
</log>
<property name="ContentType"
value="application/soap+xml"
scope="transport"
type="STRING"/>
<property name="messageType"
value="application/soap+xml"
scope="transport"
type="STRING"/>
<payloadFactory media-type="xml">
<format>
<a xmlns="">$1</a>
</format>
<args>
<arg value="my value"/>
</args>
</payloadFactory>
<send/>
</outSequence>
</target>
<publishWSDL uri="file:/C:/wso2/wso2esb-4.8.1/repository/workspaces/myproject/SCL3.wsdl"/>
</proxy>

Answer 1

I've seen that my servlet was setting content type to "text/xml" instead of "text/plain", I've changed it to "text/plain" and everything is working fine now.
Therefore I deduce that the error message
"Content is not allowed in prolog"
actually means
"Unexpected content type"

Answer 2

When you have following outsequence configuration what your log prints?
<outSequence>
<log level="full">
<property name="MyTrace" value="--- RESPONSE ---"/>
</log>
<send/>
</outSequence>