Migration has spun ubercart into checkout redirects
I had a very similar problem with Ubercart in my site as well. The difference is that I would end up getting told that there was an infinite redirect and that seem to be quite what you're hitting. However, the root cause might be the same. I had an error in an email template that was supposed to get sent after completing checkout. I would suggest checking your server logs and possibly even setting display_errors = On in your PHP config. Also check your Watchdog logs for any errors.