iOS Unity randomly hangs on yield return www

I'm developing an iOS app with Unity where I load textures at runtime from the file system. I only have one scene. The piece of code I use to load the textures is shown below. This is called several times during runtime if the user wishes to change these so called 'backTextures'.

In my tests I call this several times in a row. Everything goes well and afaik there is no sign of memory leaks. But after executing this several random times (say as little as 2 times or even as much as 13 times for example) Unity hangs.

According to the Xcode console, the last print I read is the 'Just created the www' so I am assuming the script is hanging waiting for the 'yield return _www;'. This happens always on the first iteration of the for loop (i=0), but as I said before, after calling this piece of code several times.

Can anyone help me out with this? I have no idea why yield return www would hang...

EDIT I did some more testing: This code is running on a coroutine. I added a print to the Update() and it keeps being called. So the Unity engine is not completely dead. It really seems that it is the www that is not returning....

 //some code before
 
                        if(backTextures != null && backTextures.Length != 0){
                             Debug.Log("Cleaning the vector!!!!");
                             for(int i = 0; i < backTextures.Length; i++){
                                 Destroy(backTextures[i]);
                                 backTextures[i] = null;
                             }
                             
                         }

 //Some more code here
 
 Debug.Log("Will load " + picsize + " textures");
             //load textures to memory
             for(int i=0; i<picsize; i++){
                     Debug.Log("Let's read the texture " + i);
                     string _textureURL = "file://" + Application.dataPath + "/../../Documents/current/" + imageNames[i];
                     Debug.Log("The path is " + _textureURL);
                     WWW _www = new WWW(_textureURL);
                     Debug.Log("Just created the www");
 
                     yield return _www;
                      Debug.Log("Returned from yield return");
                     if (_www.error != null)
                     {
                        Debug.LogError("WWW Error: "+ _www.error);
                         i--;
                         _www.Dispose();
                         _www = null;
                         continue;
                     }else{
                         Debug.Log("There was no error!!!!!");
                     }
                     Debug.Log("Will assign textures");
                     backTextures[i] = _www.texture;
                     Debug.Log("Will dispose textures");
                     _www.Dispose();
                     _www = null;
             }
 
 // some code after