I play on Minecraft: Windows 10 Edition, and since I updated to the new 1.12 version my friends get a simple (can't connect to world) message. However, I can join anyone's game.
I have already tried uninstalling and reinstalling, hard resetting my Xbox and router and I have checked my NAT type which is Open. If anyone has a solution, please let me know.
Be sure your default spawnpoint is not obstructed. If you want, try and place it again. You will need to enable commands for this.
I had the same problem with my friends, and that was because I had being playing in the Nether.
Be sure to have your spawnpoint and the world's spawnpoint both in the overworld (spawnpoint for the player and setworldspawn for the world, both set the spawn where you cast them). I also made sure to stay in the overwold while they entered for the first time, but that shouldn't be necessary.
