Hi MariLuz,
You can try to check Do Not Use SOAP envelope, with this check you can avoid that PI tries to deal with SOAP PI envelope.
Also, you are working with a Sync/Async bridge, depending your PI version you have some alternatives to use BPM:
- Sync/Async Bridge (http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/80f96dbf-adca-3010-ffb5-daf2d1f0e276?QuickLink=index&…). I think from PI 7.31 is available.
- You can wrap your IDOC inside a RFC in your ECC endpoint, and to do a SOAP - RFC sync scenario.
Hope this helps.
Regards.