Windows 10 Universal app: "The size of the notification group is too large"

When I call Toaster.Show it throws 0x803E0209 (WPN_E_GROUP_SIZE) "The size of the notification group is too large" exception.

I have no notification at all in the notification center.

I tried changing group name and tag but get the same error.

That's a feature not yet available on Anniversary update of Windows 10. In the Creators update the length will be upgraded to 64 characters but for now it is limited to 16.

Reference: Windows Developer Feedback