Exception type: InvalidOperationException
Source: System.Xml
Target Site: System.Object Deserialize(System.Xml.XmlReader,
System.String, System.Xml.Serialization.XmlDeserializationEvents)
The following is a stack trace that identifies the location where the
exception occurred

I was using the BizTalk orchestration xpath command, like this:

myString = xpath(myMessage,myXpath);

The problem was, that I forgot to wrap the xpath with the string() function, like this:

myString = xpath(myMessage,”string(” + myXpath + “)”);