I re-synced my achievement world to get the ghast tears and wither skulls I had been farming in the Xbox One version in order to try for the rest of the achievements that will unlock for me. My friend and I were planning to do the wither fight and two ender dragon fights so both of us would get the achievements. I was loaded into my world getting my gear together when he tried to join, but he got an "unable to connect to world" message.

So both of us signed out and tried again. Same thing. I even restarted my Xbox. It still wouldn't work.

All the settings are the same as the world I had been playing in, and that world he could join just fine. But the world I just re-synced, he can't. I even went so far as to delete and re-sync it again, and it did not make any difference.