Hello TRX,
let’s fix that payload.
Most likely cause: the toast XML you sent isn’t a valid WNS toast template for the target Windows version/registration (or it has stray characters/BOM). Try the modern ToastGeneric payload (works on Win10+ UWP) — example you can paste in the portal:
<?xml version="1.0" encoding="utf-8"?>
<toast>
<visual>
<binding template="ToastGeneric">
<text>Contoso</text>
<text>Your test message here</text>
</binding>
</visual>
</toast>
If you target older Windows/Windows Phone templates use ToastText01/ToastText02 and include the correct <text id="1"> / <text id="2"> elements. Also check: (1) registration is for Windows platform (not raw-only), (2) your Notification Hub has valid WNS credentials, and (3) the payload has no hidden BOM/extra chars and is well-formed XML.
If you want, paste the exact XML you tried and I’ll point out what’s wrong. Hope this helps — try this once.