Not sure if anybody else is having the same issue as me or not, but good information anyway.
Bought AC IV on launch day (xbox one) and was able to connect to the Ubisoft servers with 0 issues, that was until a few days ago. Since then I haven't been able to access the Ubisoft servers for single player OR multiplayer through the actual game; oddly enough I could through the uplay app on the marketplace. So, got fed up today and actually called Ubisoft (yes, there is a real phone number it's awesome, you get to talk to real people with no wait time) and we spent about 2 hours trying to fix the issue. Here's what we tried:
Unlinking and relinking xbox live gt to uplay account (5 times...)
Deleting uplay app after unlinking account and signing in through game (didn't work; couldn't talk to server...)
Delete game; reinstall.
Delete game and all DLC, reinstall game.
Delete game, all DLC, disconnect internet, hard reset console, reinstall game while disconnected from Internet, delete save game, connect to Internet, download patch.
All failed, and super fun fact - your xbox one saves for AC IV are on ubisofts servers...not MS, so 30+ hour save game also gone.
Then I called MS to see if it was an issue on their side (bad link between GT AND UBISOFT). I'm now waiting on a call back from MS level 3 support hopefully within the next 48 hours. I'll update if this gets fixed.