Well the second link works for me.
The first URL is a 404 error as you mentioned, however I'm not sure of the validity of that URL as in avast 5.0 there are no push updates in the same way as the 4.8 subscription to the push update emails. There is a different process for 5.0 pro, so I would say the second link (which relates to 4.8 iAVS push updates) is the one that should be used.
The problem being that it doesn't go to the corresponding www2. URL on Send Request, so it falls over with 404. Since this is a success response, presumably you will have been unsubscribed, just that you didn't get the success page displayed.