Hello -
same problem here (Germany).
"iTunes konnte keine Verbindung zum iTunes Store herstellen. Ein unbekannter Fehler ist aufgetreten (-9836). Überprüfen Sie, ob Ihre Netzwerkverbindung aktiv ist, und versuchen Sie es erneut."
I'm online, both WiFi and LAN work, I can use Safari etc., but when I try to access the iTunes Store or the App Store I get an error message which more or less says that the necessary internet connection could not be established.
App Store err msg: "Verbindung zum App Store nicht möglich - es ist eine Internetverbindung erforderlich."
My MBP ist a MD101D/A 2012 (bought in 2013) upgraded with 16GB RAM and a Samsung EVO 840 SSD 512GB instead of the original HDD.
OS-X Mountain Lion (10.8.5), iTunes 12.4.0.119.
Reason for my failure to upgrade the OS-X is that I had bought Parallels to run Windows 7 on the MBP for Mountain Lion, and that software would have not worked on the newer OSXs. Plus I am hesitant to upgrade to OSs which are much younger than the device it is supposed to run on; usually the device ends up being overwhelmed. But since I thought 5 years are enough to be behind I thought I should upgrade to whatever is the newest compatible OS in the App Store.
I called the Apple Technical Support and was met with extremely motivated and friendly service. We established that it does not have anything to do with my user profile or with the SSD, or with the internet access. So far we have not found a solution.
But if this started yesterday or only a few days ago maybe a solution is around the corner.