Azure notifications hub shows that from 13k WNS toast notification broadcast only 5k messages succeeded and 8k got "WNS Channel Disconnected" errors. What can cause such large amount of errors? What can I check, and why this happens?
It can be because of channel is no longer valid and should be refreshed. Push notifications are tough to debug. Could you please say what is the time period of that "13k push"?
Also check the WNS overview - it has the "Important notes" section https://msdn.microsoft.com/en-us/windows/uwp/controls-and-patterns/tiles-and-notifications-windows-push-notification-services--wns--overview
There is a useful page as well about debugging Notification Hubs: https://msdn.microsoft.com/en-us/library/azure/dn530751.aspx
So i would go with the checking if channel is even valid.