Hi, I used your method to teleport the player to another dimension and it seems that the player won't be added to World.loadedEntityList. That's a problem. I tried to add the player manually and edited parts of the method but I couldn't fix it.
Teleporting in minecraft is very hard to understand.

Yes it is... If your using 1.10 that code was a quick port from the 1.7 version. And i have not done a lot of testing with it yet.