Thanks, AndreasMalthe! Your solution finally worked for me.
I am using iOS 7.1.1 on my iPad. I was not able to connect to the App Store or iTunes. Here are all the steps I tried before yours worked:
1) I set the date forward a year then tried to sign into iTunes. I still could not sign in.
2) I set the date back to automatic. I still could not sign in.
3) I used Reset > Reset All Settings. I still could not sign in. NOTE: this still remembered my Wifi connection and password.
Then I read your post and used the Reset > Reset Network Settings. After my iPad restarted, I had to select my Wifi network and enter my password then I was finally able to login!