I think there is a bug in XMPPTLSConnection method sendXML. Method has got one parameter which should be an string. Sometimes for example while sending auth message to server this parameter is an XML object. In XMPPConnection there is a code which checks the parameter and if it is an XML it is converted to a string via toXMLString method.
Code from XMPPConnection.as:
someData = someData is XML ? XML( someData ).toXMLString() : someData;