It turns so that latest version of Unity (5.3) has stopped the support of Windows 8/Windows Phone 8, as they were deprecated in Unity 5.2. From now on Unity supports only Windows 8.1/Windows Phone 8.1 and upper versions.
Considering Unity upgrade, we updated our SDK in order to integrate with Windows 8.1/Windows Phone 8.1. It means that we won’t support Windows 8 any longer. Previously, when Windows 8 was in use, we used Microsoft Push Notification Service (MPNS) to deliver push notifications. With the advent of Windows 8.1 and Windows phone 8.1 we switched to Windows Notification Service (WNS) to stay up-to-date.
As Unity has already deprecated Windows 8, Microsoft is about to switch fully to WNS and our SDK will no longer support MPNS, we highly recommend you to upgrade as well.