logResponse will call toString() on the message, this will trigger the parsing of the xml, parsing exceptions are suppressed by toString. However the message (SaajSoapMessage) will be partially filled.
When getEnvelope() is called the incomplete message will be returned without any error or exception.
This issue does not seem to exist anymore in the current 1.5.9 SVN codebase. I get the following exception when the message is loaded:
org.springframework.ws.soap.saaj.SaajSoapMessageException: Could not access envelope: Unable to create envelope from given source: ; nested exception is com.sun.xml.internal.messaging.saaj.SOAPExceptionImpl: Unable to create envelope from given source: