XMLCoder decode invalid data type comwmlangxmlDocument


when my customer send me purchase order that begin with:

SAP business Connector 4.6 SR1 (webMethods Integration Server 4.6 embeded) produce erroe in server log:
[B2BCORE.0076.0007] XMLCoder decode invalid data type: com.wm.lang.xml.Document
and doesn’t parse xml document send by http protocol.
But, when I test sending same document but with modified first line:

everything works fine. Problem is that first line is generated automaticaly and this worked till a few days ago.

Please help