This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
The most likely cause of the problem is that your action does not produce valid XML. For example, the statement:
clearly produces invalid XML. I'd suggest having your action class produce a System.out.println of the final result to be returned. Then cut and paste that string that is output into a file and see if Internet Explorer can read the document without errors. Then you know that at least you have valid XML.