Soundcloud API /resolve method 404 error
So after some testing I've confirmed that the issue is related to the fact that you're trying to request a private set. Because the set is private, you would need to first go through SoundCloud's OAuth 2 authentication process before being able to access the set.
Try resolving the set url with the url found in the share dialog of your set. This url contains a secret token that can be used to access a private set without logging in.
Not sure how far this'll get you if the tracks in the set are also private, though. And like bbone mentioned, you'll be exposing this secret token, which may or may not be a problem.