Asp.net core 2.0 TLS 1.2 exception "The client and server cannot communicate because they do not possess a common algorithm"

Question

Let me list specs regarding this application:

• Windows 10 for all of us
• Asp.net core 2.0 web app
• We all use VS2017
• We're using IIS Express
• SSL is turned on for this application
• We trust our IIS express localhost cert

During startup we do the following:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

The problem we're having is only happening to some of our team members in their localhost environment.

In a background task thread, we use the System.Net.WebClient to make a call to a different URL of our app (again, to localhost), but it results in an exception whose innermost exception message is

The client and server cannot communicate, because they do not possess a common algorithm.

We know that setting ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls; fixes the problem but we need it to remain at Tls12.

This must be a setting in either IIS Express or a firewall setting or something else environmental.

Some posts on Google suggest that we install .NET 4.6+ but we all have it installed already, so that's not it.