Unity Completely Freeze during LoadLevelAsync

Hi Everyone! I'm having some troubles with unity.

i was working on a project made on unity 4, and our client need us to send the game with unity 5

when we ported our game we faced a lot of bugs and troubles that we are working on, but one i dont seem to fix or find why its happening is a complete freeze on unity editor when i load a new scene.

my code is the following:

Code (CSharp):

1. public class LoadGameplayAsync : MonoBehaviour {
2.  
3.     AsyncOperation gameplayLoading;
4.  
5.     // Use this for initialization
6.     void Start () {
7.         StartCoroutine("loadGameplay");
8.         gameplayLoading.allowSceneActivation = true;
9.     }
10.  
11.     IEnumerator loadGameplay()
12.     {
13.         gameplayLoading = Application.LoadLevelAsync("GamePlay Tokio");
14.         gameplayLoading.allowSceneActivation = false;
15.         yield return gameplayLoading;
16.     }
17. }

After this Load Async, unity stop responding and i have to kill the proces.

Note: This happens randomly, sometimes it freeze, some times it load normally.

Any hints on this would be really appreciated!

Extra Details: This script is attached to the main camera on an empty scene, that contains a canvas and a simple "loading animation"