Looking at the code for Microsoft.XLANGs.Core.Part.GenericLoadFrom(Object source) it will throw a “Specified method not supported error” only if the source argument is a System.Xml.XmlNodeList with more than 1 XmlNode. This is called from Message_1 = xpath(SQLMessage, Variable_3);
I suspect the xpath function is returning more than one node. What is the HeaderInstance variable and how is it assigned?