Looks like problem is based on a server side.
Im my case I worked with paypal server and neither of suggested answers helped, but http://forums.iis.net/t/1217360.aspx?HTTP+403+Forbidden+error
I was facing this issue and just got the reply from Paypal technical.
Add this will fix the 403 issue.
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
req.UserAgent = "[any words that is more than 5 characters]";