Hi
I have just had the same issue. I uploaded an app a few days ago that worked fine. Then I uploaded a similar app yesterday and followed the same processes as before, but now I'm getting the 'Authentication is required...' message when I try to download it. This is only happening to my app. I can download anything else from the Play Store.
I am busy unpublishing the app and am going to upload a fresh copy to see if that works. One question. The Private Sign key, it was stored in the folder of a different app that I have created, but used for the one I am having trouble publishing. Could this be a factor?