In the file NGUITools.cs File.ReadAllBytes(path) is used.
This seemed to work fine with Unity 4.2 but with 4.3 it doesn't anymore.
So to fix it the code needs to be adjusted to the following:
#if UNITY_WEBPLAYER || UNITY_FLASH || UNITY_METRO || UNITY_WP8
return null;
#else
if (!NGUITools.fileAccess) return null;
string path = Application.persistentDataPath + "/" + fileName;
if (File.Exists(path))
{
return File.ReadAllBytes(path);
}
return null;