Since I have upgraded OS X on my MacBook Pro to OS X El Capitan (10.11) and moved my users home folder to another partition as described in an earlier blog post, I was not able to sign into the Apple or iTunes store anymore via native applications. It was still working fine via the browser. The failure displayed was: “AMD-Action:downloadProduct:SP”. Yes, very helpful. No further link for help nor was I able to find anything about it on official support pages.
Querying the web for a while I finally found the solution for my case. The reason here was that my home directory is not on the primary volume. If you’re in such a situation you can easily fix that by creating a new folder (/Users/Shared) which will be located on the primary volume. Give it access to everyone, and voila it will work. For detailed steps in doing that read this c|net article.
If that doesn’t help you it might also be related to some configuration failures in your network settings. To fix that follow those steps