I was simply trying to update my installed apps. The problem hit me simultaneously on iPhone and iPad — which is a hint that the problem is server side.
No amount of waiting to try again later made a difference. Not resetting the time. Nor restarting my iPhone/iPad. Nor rebooting my iPhone/iPad. Nor deleting a couple of videos to recover memory (even though I wasn’t short on available RAM to start with). Nor even switching between 3G and WiFi, thereby excluding router issues.
For me the solution was different and simple.
I needed to sign out of my iTunes/App Store account and back in again …
1. Click on your Apple ID
2. Sign Out
1. Sign in
2. Use Existing Apple ID
1. Enter your iTunes email (username) and password
2. Click Ok
Restore your settings — or make new choices.