How to fix 'The underlying connection was closed: An unexpected error occurred on a send'

This issue can be related to several factors, such as SSL/TLS protocol mismatches, server configuration issues, or network problems.

To solve the problem you should set the configuration as the following c# code.

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12;

You can add the code above in the Program class.

static class Program
{  
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main()
    {        
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12;
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new frmFluentDesignForm());
    }
}

I hope so you can solve the problem when using RestSharp library with https protocol.