So you're saying that things are working if you return null from you action method, but not if you return a string like "success"? I've read somewhere, I guess it's in the developer handbook, that when doing AJAX requests, you should _alway_ return null.
Well you are absolutely right, seems I have missed this paragraph in 5.5.4 of the Developer Guide. It explains very well the effect that I had. With a non-null return value the full view was re-rendered, no partial update occurred. Thanks for pointing me there!