Since the problem only occurs in IE6 while running on the production server, it might has to do with IE6's buggy gzip support. Is there a way to test this by turning off gzip compression on the server?
If you're using jetty to serve static data you can disable it in the framework via the "UseGzip" configuration setting (it's a boolean)
Where does it freeze? As soon as you open it or when going to a page with the navigation tree in it? (If these are the same page, try setting the Navigation Tree on a different page and try again).
Edit: Is the Navigation Tree widget on your homepage?
Where does it freeze? As soon as you open it or when going to a page with the navigation tree in it? (If these are the same page, try setting the Navigation Tree on a different page and try again).