Just got redirected here from a semi-related but different bug. Cannot access Realms after leavint the Minecraft Beta on Xbox. (MCPE-113102)
No, listen, this is a different bug. This is after leaving the Minecraft Beta. That ticket is due to a bad connection to the XBL network/auth servers. Please do not mark this as resolved.
Update: Problem solved. The person I share Xbox Live with got signed out of my console. No idea how, and I play multiplayer games so rarely I never noticed. A more detailed error message would be appreciated though. 👍