The following exception is thrown intermittently when calling GetSalesInvoiceByKey:
Token StartElement in state EndRootElement would result in an invalid XML document. Make sure that the ConformanceLevel setting is set to ConformanceLevel.Fragment or ConformanceLevel.Auto if you want to write an XML fragment.
It appears, through searching on the exception, this was an issue in previous versions but the indications are it was to be fixed. This is only happening intermittently and causing a great deal of frustration because the invoice being passed in as the key is valid and nothing is very specific regarding the error message. Not sure where to go from here; the reliability of this call or the web service in general appears to be suspect at best.