Error in Call Adapter (IDOC->XI->FILE)

Hello,

i have got the following error message in the integration engine monitoring in the "call adapter" part:

td {font-family : Arial, Tahoma, Helvetica, sans-serif; font-size : 14px;} A:link A:visited A:active HTTP response contains status code 503 with the description Service Unavailable XML element Envelope missing in SOAP message header (SAP XI Extension)

What is the AFW application?????