Hello,
I have (almost) the same problem on an iPhone 6S. These are the symptoms:
- can't update any apps - when i tap "Update", the update process get stuck at the very beginning (With the stop sign in the middle, without any progress)
- can't download new apps - as above
- Settings > General > Software Update tells me "Unable to Check for Update"
- Meanwhile the system is slow, sometimes sluggish, sometimes I have to wait seconds after launching an app (e.g. Facebook, Outlook, Tumblr, even Settings) to start
- the phone itself sometimes gets very hot
- also it drains the battery down REALLY fast. (Sometimes it can be 1 percent per minute while i am not doing anything on the phone)
What I've tried, without any luck:
- restarting the phone
- Reset network settings
- Reset all settings
- force restart the phone (with keeping home + lock pressed for 10 or so seconds)
- sign out and sign in into the app store
- sign out and sign in into iCloud (just to be sure)
- killing the App Store (with removing it from the stack)
- refreshing App Store (with pressing the Updates button at the bottom righ corner 10 or so times quicky)
- all of the above with different order - many times
- also tried to restore the phone - without any of my data it seemed to work well, but when i restored the backup, it just got into the same state
I have a feeling that this all is somewhat related to my photos. The Photos app keeps doing something in the background when I open it, despite the fact, that it already scanned all of my photos, and uploaded back all of them into iCloud. Also, the Photos app can be extremely slow - after selecting someone in the People album, it can be 5-15 seconds to actually get into that folder.