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Same issue here.
Last time, I reset all settings, but it only helped for a day or two.

Hoping for an easier and less annoying fix, ideally a new beta today or tomorrow :)

(Haven’t tried installing apps via iTunes. May try today.)
 
Resetting network settings didn’t help for me. I had to use iTunes on a Mac to add an app. Just tested it now from the iOS App Store and downloading apps is working fine again.
 
Still not working for me on iPhone. Can't download any apps on iOS 11 beta 3.

Think it stopped working on Friday/Saturday
 
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