www 404 error only on iOS

Hi

I have a problem with WWW, only on iOS device. trying to download a simple plain text-file from web server.

The URL looks like this.. http://a.b.c/d_e/0.1.2/target.txt
It has alphabetic characters and some special characters only ( : / . _ ).
Has no blank space, etc. and hard-coded.

And my script(pseudo code) is..

     IEnumlator Start()
     {
        Debug.Log( url );
        using( WWW www = new WWW(url) )
        {
           if ( false == www.isDone )
           {
              Debug.Log( "yield.." );
              yield return null;
           }
 
           if ( null != www.error )
           {
              Debug.Log( www.error );
           }
           else
           {
              Debug.Log( "SUCCEED!" );
           }
       }
    }

Have tested..

1. Windows7 + unity editor 4.6.1p5
   

2. OSX Mavericks + unity editor 4.6.2

3. OSX Yosemite + unity editor 4.6.3

4. Safari, Chrome, IE...

5. Android device (4.3)

6. iOS device (7.x)

case 1,2,3,4,5 they all show me the "SUCCEED!" on log. ( Web browser display the contents of target file. )
but, "404 Not Found" only on ios ...

All log looks like below on ios

---

http://a.b.c/d_e/0.1.2/target.txt
yield.. <-- just one time.
404 Not Found

---

...

What i have missed?

Thanks.