Cannot connect to App Store due to unactivated SIM? I’ve picked up my new iPhone 6 yesterday. I’ve set up my Apple ID and connected it to my WLAN normally. Internet works, but when I launch the App Store it just says “Cannot connect…”.

Could it be due to the unactivated SIM card? The phone is locked to T-Mobile Croatia, but they haven’t yet activated my SIM card. (I expect them to do this throughout today, but there could be delays.)

The only other cause I can think of is my preexisting Apple ID. I’ve created it last year when I wanted to access some Apple developer specific videos, but I haven’t had an Apple device until now.
Turns out, iCloud Drive (or a related service) messed up my Apple ID in some way. I signed out of my Apple ID, performed a password reset, and signed in using the new password. Now everything seems to work fine.