Transfer closed with bytes remaining to read

To download packages from server I use code like this

 void SomeFunction()
 {
        StartCoroutine(DownloadPackage());
 }
 
 
 IEnumerator DownloadPackage()
 {
     WWW www = new WWW("http://someserver.com/package.zip");
     yield return www;
     byte[] package = www.bytes;
     File.WriteAllBytes("Some/Directory/package.zip",package);
 }

And I have a problem!

Almost always i have an error

"You are trying to load data from a www stream which had the following error when downloading. transfer closed with N bytes remaining to read"

I try to find problem! What it could be? Or it is problem in Unity, or it is problem in internet connection, or server problem!

I try to download this package using web browser and everything good! So I think, that server works perfectly!

So, how to solve this problem? And is there any alternative methods to download files from internet?