Generated sprite not visible on iOS

Hi, i'm downloading an image from web via WWW. That works perfect for all targeted platforms excepting iOS. On iOS the image appears just black.

Here is the code:

 public void receiveData(WWW receivedData)
 {
     image.sprite = Sprite.Create(receivedData.texture, new Rect(new Vector2(0,0), new Vector2(50, 50)), new Vector2(0.5f, 0.5f));
     image.color = Color.white;
 }

The method is called by a coroutine that waits for the WWW and handels some errors.

I've been trying around for some time now without any results... e.g. i tried to change the textureFormat or creating a new Texture2D and changed some pixel. But all results in a black image. Does anyone have an idea what the matter is?

Best Regards