I suggested Java 7 for a very strong reason: the life cycle of Java 6 ended in February 2013 therefore updates, fixes and support for that version have ceased. Running mission critical systems on unsupported version is a high and unnecessary risk no company should take.
Short considereation before you go for a deep dive into Java troubleshooting: reiterating my previous post, that is unrelated to Marketo. Oracle Global Support would be your best resource. I have been on that side for several years.
That error can range from simple malformed XML to convoluted Java configuration or conflictling SSL library versions.
ParseError at [row,col] means the response could not return any result. That is not zero record, that is really null.
You would start double checking whether the endpoint is correct. In the past it was similar to http://app.marketo.com/soap/mktows/2_2?WSDL
The new ones have the Munchkin ID embedded such as
https://XXX-YYY-ZZZ.mktoapi.com/soap/mktows/2_2
It may an attempt to read data before it is actually written. That happens when multiple threads are used (Tomcat).
There are valuable Oracle documents to help you at http://docs.oracle.com/cd/E17802_01/webservices/webservices/docs/1.6/tutorial/doc/SJSXP4.htmlhttp://www.oracle.com/technetwork/java/stax-139959.htmlhttp://docs.oracle.com/javaee/5/api/javax/xml/stream/XMLStreamReader.html