Asp.net core website intermittently refusing connections - asp.net-core

Question

I have an asp.net core 3.0 website. It has a controller that implements an HttpGet function that does some database stuff then returns a Json object (https://localhost:44356/api/runner/match).
I have a console application that uses an HttpClient to hit that url. I launch the site locally and then I launch my console app. About 50% of the time it works. The other 50% of the time I get:
HttpRequestException: No connection could be made because the target machine actively refused it.
SocketException: No connection could be made because the target machine actively refused it.
I'm trying to figure out why my console app's connection is being blocked. I don't know how to start debugging this. I tried to implement a retry on the request, but once I get the exception, I keep getting it. So I think it's something non-deterministic happening in my website, potentially related to SSL?
I'm able to hit the url in Chrome locally just fine.
How do I figure out what is blocking the connection from being made?
Is there any chance this is something IIS Express is doing?
Calling code in console app:
static async Task<List<Deck>> GetMatchData()
{
string baseUrl = "https://localhost:44356/api/runner/match";
using (HttpClient client = new HttpClient())
{
HttpResponseMessage res = null;
res = await client.GetAsync(baseUrl);
Controller function:
[HttpGet("match")]
public async Task<ActionResult> GetMatchup()
{
int count = db.Decks.Count();
Random r = new Random();
int d1 = r.Next(count) + 1; // database ids start at 1 for some reason
int d2 = r.Next(count - 1) + 1;
if (d1 == d2)
d2++;
List<Deck> result = new List<Deck>();
result.Add(await db.Decks.FindAsync(d1));
result.Add(await db.Decks.FindAsync(d2));
if (result[0] == null || result[1] == null)
{
return BadRequest();
}
return Ok(result);
}

Answer 1

try
HttpClient httpClient = new HttpClient();
//specify to use TLS 1.2 as default connection
System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
string baseUrl = "https://localhost:44356/api/runner/match";
httpClient.BaseAddress = new Uri(baseUrl );
httpClient.DefaultRequestHeaders.Accept.Clear();
httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
var getResult = await httpClient.GetAsync(baseUrl);