"Internal server error" when trying to read RSS feed with WebClient

Im trying to read an RSS feed with WebClient to a string variable, but I keep getting "The remote server returned an error: (500) Internal Server Error.". The loginname and password are correct since othewise I would get "401: unauthorized".
Heres what I do:

WebClient client = new WebClient();

NetworkCredential credential = new NetworkCredential("loginname", "password");

client.Credentials = credential;

string str = client.DownloadString("http://www.addresstotherssfeed.com");
What can be causing this error? I am able to read the RSS feed with Feed Demon and even in my browser if I put the "http://www.addresstotherssfeed.com" to the address bar..
 

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