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WebException: The request was aborted: Could not create SSL/TLS secure channel but working with POSTMAN

I am calling Power school API to fetch student data. Using the below code.
public Token GetAuthToken(string baseUrl, string authKey)
{
try
{
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls13;
var client = new HttpClient();
client.BaseAddress = new Uri(_url);
client.DefaultRequestHeaders
.Accept
.Add(new MediaTypeWithQualityHeaderValue("application/json"));
var values = new Dictionary<string, string> { { "grant_type", "client_credentials" } };
var content = new FormUrlEncodedContent(values);
client.DefaultRequestHeaders.Add("Authorization", $"Basic {authKey}");
var request = new HttpRequestMessage
{
Method = HttpMethod.Post,
Content = content,
};
request.Headers.Add("Authorization", $"Basic {authKey}");
var response = client.PostAsync(baseUrl+ "/oauth/access_token", content).Result;
var responseString = response.Content.ReadAsStringAsync().Result;
var token = JsonConvert.DeserializeObject<Token>(responseString);
return token;
}
catch (Exception e)
{
throw;
}
finally
{
}
}
This code was working fine 3 months ago but now started giving error WebException: The request was aborted: Could not create SSL/TLS secure channel.
I tried to post the same request from POSTMAN and it was able to connect and get data. Postman shows TLS 1.2. See the below Image
I also tried the code generated from the POSTMAN and it was giving the same error. Below is the code generated from POSTMAN
var client = new RestClient("<MY URL HERE>");
client.Timeout = -1;
var request = new RestRequest(Method.POST);
request.AddHeader("Authorization", "Basic <MY KEY HERE>");
request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
request.AddParameter("grant_type", "client_credentials");
IRestResponse response = client.Execute(request);
Console.WriteLine(response.Content);
I am using these on Windows Server 2012 R2. POSTMAN is able to connect while C# (.Net Framework 4.8) is failing.
What could be the issue? I tried a combination of all TLS versions available from TLS to TLS13
EDIT
When I check event logs, I am getting below error

RestClient Digest Authentication C#

I am trying to make a post request using RestClient with Digest Authentication .
But I keep getting error :
Cannot load all required data from authenticateHeader. Data: Realm="api domain"&Nonce="7fe0990c20aaa812c5a0a725e3a01423"&Qop="""
My code
var client = new RestClient("ApiUrlPath");
client.Authenticator = new DigestAuthenticator("username","Password");
client.UseUtf8Json();
client.Timeout = -1;
var request = new RestRequest();
request.AlwaysMultipartFormData = true;
request.AddParameter("id", "2");
var response = client.Execute(request);
The request works in postman

How to post a Multipart/mixed:boundary=”MIME-Boundary” with two different document types and HMAC signature in C# [Open Invoice]

As part of an integration to Open Invoice the API is requiring a legacy Multipart/mixed:boundary=”MIME-Boundary” request header. There is very minimal documentation on utilizing this Header for C# across the internet including Microsoft documentation.
The two documents in the POST request include a UTF-8 XML string and a Base64 pdf string. The outbound request requires Content-Type headers (one for each document) which HTTPRequestMessage does not support natively as it assumes you will be delivering a "text/plain" request.
Headers.Add() //throws an exception when setting Content-Type
Headers.TryAddWithoutValidation() //does not fix the problem
Additionally the request needs to be signed with an HMAC hash of the request body.
How do I build this request in C#?

Build the client
X509Certificate2 cert = new X509Certificate2(certificateName, certPassword);
var handler = new WebRequestHandler();
handler.ClientCertificateOptions = ClientCertificateOption.Manual;
handler.ClientCertificates.Add(cert);
HttpClient client = new HttpClient(handler);
client.BaseAddress = new Uri(openInvoiceURL);
Build The Request
//Starting from byte[] for both files
var xmlString = System.Text.Encoding.UTF8.GetString(xmlBytes);
string pdf64 = Convert.ToBase64String(pdfBytes);
string url = "/docp/supply-chain/v1/invoices";
var multiPartContent = new MultipartContent("mixed", "_MIME-Boundary");
var xmlHttpContent = new StringContent(xmlString);
xmlHttpContent.Headers.ContentType = new MediaTypeHeaderValue("application/xml");
multiPartContent.Add(xmlHttpContent);
var pdfHttpContent = new StringContent(pdf64);
pdfHttpContent.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");
pdfHttpContent.Headers.Add("content-transfer-encoding", "base64");
pdfHttpContent.Headers.Add("Content-ID", pdfName);
multiPartContent.Add(pdfHttpContent);
var request = new HttpRequestMessage(HttpMethod.Post, url);
request.Content = multiPartContent;
request = AttachHMAC(request, secretKey);
var response = client.SendAsync(request).Result;
var responseString = response.Content.ReadAsStringAsync().Result;
Sign the Request Body (only on POST)
private HttpRequestMessage AttachHMAC(HttpRequestMessage request, string secretKey)
{
var payload = string.Empty;
if(request != null && request.Content != null)
{
payload = request.Content.ReadAsStringAsync().Result;
}
var hashMaker = new HMACSHA256(Encoding.UTF8.GetBytes(secretKey));
var payloadByteArray = Encoding.UTF8.GetBytes(payload);
byte[] hash = hashMaker.ComputeHash(payloadByteArray);
var base64Hash = Convert.ToBase64String(hash);
request.Headers.Add("mac", base64Hash);
return request;
}

SharePoint 2010 ListData.svc REST API Randomly returns error: 'uriString' parameter represents an absolute URI from HTTPClient

I'm experiencing an intermittent problem with our SharePoint 2010 REST API. I have a .Net Core Console application that makes a series of calls to SharePoint List Endpoints to get a JSON response. My problem is that at random times, the API response is an error page:
A relative URI cannot be created because the 'uriString' parameter
represents an absolute URI.http://www.example.com/somefolder/file.svc
Is there a problem with my HTTPClient configuration? Is there a configuration setting that I can toggle in SharePoint to prevent the error or more reliable?
var uri = new Uri("http://www.example.com/");
var credential = new NetworkCredential("username", "password", "domain");
var credentialsCache = new CredentialCache { { uri, "NTLM", credential } };
var handler = new HttpClientHandler { Credentials = credentialsCache };
HttpClient Client = new HttpClient(handler);
Client.BaseAddress = new Uri("http://www.example.com/sharepoint/path/ListData.svc/");
// Make the list request
var result = await Client.GetAsync("MySharePointList");

To get the list items, the REST API URI like below.
http://sp2010/_vti_bin/ListData.svc/listname
Modify the code as below.
var siteUrl = "http://www.example.com/";
var listName = "MySharePointList";
var uri = new Uri(siteUrl);
var credential = new NetworkCredential("username", "password", "domain");
var credentialsCache = new CredentialCache { { uri, "NTLM", credential } };
var handler = new HttpClientHandler { Credentials = credentialsCache };
HttpClient client = new HttpClient(handler);
client.BaseAddress = new Uri(uri, "/_vti_bin/ListData.svc");
client.DefaultRequestHeaders.Add("Accept", "application/json;odata=verbose");
client.DefaultRequestHeaders.Add("ContentType", "application/json;odata=verbose");
var requestURL = siteUrl + "/_vti_bin/ListData.svc/" + listName;
// Make the list request
var result = client.GetAsync(requestURL).Result;
var items= result.Content.ReadAsStringAsync();

facebook error An active access token must be used to query information about the current user

I am using Facebook C# SDK with RestClient and RestRequest Classes, in order to connect to facebook.
But every few calls I am getting the following error from facebook:
An active access token must be used to query information about the
current user
this is my code:
var client = new RestClient { Authority = "https://graph.facebook.com/v2.1/oauth/" };
var request = new RestRequest { Path = "access_token" };
request.AddParameter("client_id", Storage.FacebookClientId);
request.AddParameter("redirect_uri", CallbackUrl);
request.AddParameter("client_secret", Storage.FacebookApplicationSecret);
request.AddParameter("code", code);
RestResponse response = client.Request(request);
// A little helper to parse the querystrings.
StringDictionary result = ParseQueryString(response.Content);
sAccessToken = result["access_token"];
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls;
var client = new RestClient { Authority = "https://graph.facebook.com/v2.1/" };
var request = new RestRequest { Path = "me" };
request.AddParameter("access_token", sAccessToken);
RestResponse response = client.Request(request);
Dictionary<string, object> result = _serializerService.Deserialize(response.Content);
Any Ideas please?