I finally found the answer (I haven’t noted my source but it was from a search);
While the code works in Windows XP, in Windows 7, you must add this at the beginning:
// using System.Net; ServicePointManager.Expect100Continue = true; ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; // Use SecurityProtocolType.Ssl3 if needed for compatibility reasons
And now, it works perfectly.
As mentioned by Robin French; if you are getting this problem while configuring PayPal, please note that they won’t support SSL3 starting by December, 3rd 2018. You’ll need to use TLS. Here’s Paypal page about it.