It is essential that the Name and Publisher attributes of the Identity element in the manifest exactly match what’s listed on Dev Center. An id based on these is submitted to Xbox Live during sign-in, and used to look up the Xbox Live settings (title id and scid) that the service then verifies your client against. The error you originally posted is commonly seen when that look up fails, hence my suggestion to verify your package identity against Dev Center. It sounds like there was actually a problem there, and that now the error has changed? Have you tried with the newer SDK since resolving the error? I still think that may be relevant since the SDK referenced by GitHub Head actually predates the Xbox Live Creators Program.
Title id and scid don’t feature in the manifest. They come in exclusively via the xboxservices.config file, and it sounds like that has the proper content.