Ok, I still don’t know exactly where the issue lies, because while it could be a firewall issue it could also be a Windows Update issue causing older version of security protocols to become obsolete.  I suppose it could even be a back-end update on your website that would break older versions of VisualGDB, but I suspect there would be a lot more complaints on the forums if that were the case.

This code fails:

WebClient webClient = new WebClient();
string sData = webClient.DownloadString("https://visualgdb.com/hwsupport/catalog?version=latest");

This code succeeds:

WebClient webClient = new WebClient();
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
sData = webClient.DownloadString("https://visualgdb.com/hwsupport/catalog?version=latest");

Tls12 is only available starting in .Net framework 4.5 (along with Tls11, which I also tried and it failed).  3.5 and 4 only have Tls available.