If you use Java it is a no-brainer: GWT. You can check out our app for an example: http://demo.teampostgresql.com. It is a fairly complex AJAX app written 99% in plain Java. GWT automatically compiles the source to JavaScript targeting different browsers. Awesome.
Nasty. I haven't seen it before even though the app sees a fair amount of use. Do you have the complete stack trace? Any unusual browser / configuration?