Is the App Store or iTunes Store app on your iPhone repeatedly prompting your for your Apple ID and password? I mean repeatedly as in, you enter it, the iPhone thinks a moment, and then asks you again, never satisfied.
In iOS 8, the solution to this was pretty easy: You could sign out of your account in "App and iTunes Stores", reboot, and sign back in no problem. Starting in iOS 9, a few more settings share the same configuration file that the store apps use, making the solution a little more involved.
How to get the App Store to accept your Apple ID and password:
First, we sign out of everything that holds a reference to this database file:
Settings → App and iTunes Stores → Tap your Apple ID → Sign Out
Settings → Music → Scroll down to Home Sharing at the bottom → Tap your Apple ID → Sign Out
Settings → Videos → Tap your Apple ID under Home Sharing → Sign Out
Settings → Game Center → Tap your Apple ID → Sign Out
Next, we sign out of iCloud because it seems to be needed too.
- Settings → iCloud → Scroll to bottom → Sign Out.
Here you will be prompted about whether you want to keep or delete iCloud synced data. I choose to delete the iCloud synced stuff from my iPhone, since it will come back when I sign back in. Do whatever you are comfortable with.
Now, we need to really make sure we let go by doing:
- Settings → General → Reset → Reset All Settings
Wait for the iPhone to reboot.
Raise your eyebrows at the barrage of iMessages after the phone reboots if you get them... Now sign back into things.
Settings → Wi-Fi → Sign back into your wifi. This is critical if you don't have cell service, but nice either way.
Settings → App and iTunes Stores → Sign In
Settings → iCloud → Sign In
Settings → Music → Sign back into home sharing if you want
Settings → Videos → Sign back into home sharing if you want
Settings → Game Center → Sign back in if you want