If you can't connect to the App Store, iTunes Store, or Apple Books - https://support.apple.com/HT201400
Resolve issues between iTunes and third-party security software - https://support.apple.com/en-us/HT201413 - You can’t connect to iTunes Store to buy or download content.
Please note, this is the German language community for questions and replies in German. For additional assistance in English please post in the English language community. You will get a more speedy response, in English, from a larger community that's active 24/7. --> https://discussions.apple.com