Error establishing SSL/TLS connection despite having registry keys DontEnableSchUseStrongCrypto and DontEnableSystemDefaultTlsVersions set - ssl

Question

I developed an outlook plugin which makes a call to an url to retrieve a token.
As always, "works on my machine" (OutlookVersion "16.0.0.15726", Microsoft Windows 10 Pro
Version 10.0.19044 Build 19044).
We have a test server where i tried to install the Addin for testing (OutlookVersion "16.0.0.4266", Microsoft Windows Server 2016 Standard
Version 10.0.14393 Build 14393).
When making the call with HttpClient, i get the "couldn't establish SSL/TLS connection error". Code is as follows:
protected async Task<string> ExecuteAuthRequestAsync(string authUrl, CancellationToken cancelToken, HttpClient httpClient)
{
cancelToken.LogAndThrowIfCanceled(logger);
var request = new HttpRequestMessage(HttpMethod.Get, authUrl);
request.Headers.Add("Accept", VndPaosMimetype);
var response = await httpClient.SendAsync(request, HttpCompletionOption.ResponseContentRead, cancelToken);
response.EnsureSuccessStatusCode();
var authResponse = await response.Content.ReadAsStringAsync();
return authResponse;
}
The registry keys for SchUseStrongCrypto and SystemDefaultTlsVersions are set to 1 under HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft.NETFramework\v4.0.30319 and
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft.NETFramework\v4.0.30319
(see https://learn.microsoft.com/en-us/dotnet/framework/network-programming/tls)
I also set the configuration in the .config.
I even added the diagnostics configration to the .config to trace the TLS error
{
Accept: application/vnd.paos+xml
Host: <url>
Connection: Keep-Alive
} werden gesendet.
DateTime=2022-11-24T08:53:41.6113986Z
System.Net Information: 0 : [19444] SecureChannel#31352595::.ctor(hostname=<url>, #clientCertificates=0, encryptionPolicy=RequireEncryption)
DateTime=2022-11-24T08:53:41.6164042Z
System.Net Information: 0 : [19444] Sicherheitspakete werden enumeriert:
DateTime=2022-11-24T08:53:41.6194210Z
System.Net Information: 0 : [19444] Negotiate
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] NegoExtender
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] Kerberos
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] NTLM
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] TSSSP
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] pku2u
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] CloudAP
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] WDigest
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] Schannel
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] Microsoft Unified Security Protocol Provider
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] CREDSSP
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] PWDSSP
DateTime=2022-11-24T08:53:41.6204064Z
System.Net Information: 0 : [19444] SecureChannel#31352595 - Noch 0 Clientzertifikate zum Auswählen verfügbar.
DateTime=2022-11-24T08:53:41.6294061Z
System.Net Information: 0 : [19444] SecureChannel#31352595::.AcquireClientCredentials, new SecureCredential() (flags=(ValidateManual, NoDefaultCred, SendAuxRecord, UseStrongCrypto), m_ProtocolFlags=(Zero), m_EncryptionPolicy=RequireEncryption)
DateTime=2022-11-24T08:53:41.6304013Z
System.Net Information: 0 : [19444] AcquireCredentialsHandle(package = Microsoft Unified Security Protocol Provider, intent = Outbound, scc = System.Net.SecureCredential)
DateTime=2022-11-24T08:53:41.6324058Z
System.Net Information: 0 : [19444] InitializeSecurityContext(credential = System.Net.SafeFreeCredential_SECURITY, context = (null), targetName = <url>, inFlags = ReplayDetect, SequenceDetect, Confidentiality, AllocateMemory, InitManualCredValidation)
DateTime=2022-11-24T08:53:41.6354063Z
System.Net Information: 0 : [19444] InitializeSecurityContext(Länge von In-Buffer = 0, Länge von Out-Buffer = 153, zurückgegebener Code = ContinueNeeded).
DateTime=2022-11-24T08:53:41.6414064Z
System.Net.Sockets Verbose: 0 : [19444] Entering Socket#51781231::BeginSend()
DateTime=2022-11-24T08:53:41.6464074Z
System.Net.Sockets Verbose: 0 : [19444] Exiting Socket#51781231::BeginSend() -> OverlappedAsyncResult#28002689
DateTime=2022-11-24T08:53:41.6474075Z
System.Net.Sockets Verbose: 0 : [5000] Data from Socket#51781231::PostCompletion
DateTime=2022-11-24T08:53:41.6514042Z
[...]
System.Net.Sockets Verbose: 0 : [5000] Entering Socket#51781231::EndSend(OverlappedAsyncResult#28002689)
DateTime=2022-11-24T08:53:41.6574040Z
System.Net.Sockets Verbose: 0 : [5000] Exiting Socket#51781231::EndSend() -> Int32#153
DateTime=2022-11-24T08:53:41.6584040Z
System.Net.Sockets Verbose: 0 : [5000] Entering Socket#51781231::BeginReceive()
DateTime=2022-11-24T08:53:41.6604081Z
System.Net.Sockets Verbose: 0 : [5000] Exiting Socket#51781231::BeginReceive() -> OverlappedAsyncResult#25961440
DateTime=2022-11-24T08:53:41.6624050Z
System.Net.Sockets Verbose: 0 : [5000] Data from Socket#51781231::PostCompletion
DateTime=2022-11-24T08:53:41.6624050Z
System.Net.Sockets Verbose: 0 : [5000] 00000000 : 15 03 03 00 02 : .....
DateTime=2022-11-24T08:53:41.6624050Z
System.Net.Sockets Verbose: 0 : [5000] Entering Socket#51781231::EndReceive(OverlappedAsyncResult#25961440)
DateTime=2022-11-24T08:53:41.6644056Z
System.Net.Sockets Verbose: 0 : [5000] Exiting Socket#51781231::EndReceive() -> Int32#5
DateTime=2022-11-24T08:53:41.6644056Z
System.Net.Sockets Verbose: 0 : [5000] Entering Socket#51781231::BeginReceive()
DateTime=2022-11-24T08:53:41.6664055Z
System.Net.Sockets Verbose: 0 : [19444] Data from Socket#51781231::PostCompletion
DateTime=2022-11-24T08:53:41.6664055Z
System.Net.Sockets Verbose: 0 : [19444] 00000000 : 02 28 : .(
DateTime=2022-11-24T08:53:41.6664055Z
System.Net.Sockets Verbose: 0 : [19444] Entering Socket#51781231::EndReceive(OverlappedAsyncResult#14684281)
DateTime=2022-11-24T08:53:41.6664055Z
System.Net.Sockets Verbose: 0 : [19444] Exiting Socket#51781231::EndReceive() -> Int32#2
DateTime=2022-11-24T08:53:41.6664055Z
System.Net.Sockets Verbose: 0 : [5000] Exiting Socket#51781231::BeginReceive() -> OverlappedAsyncResult#14684281
DateTime=2022-11-24T08:53:41.6664055Z
System.Net Information: 0 : [5000] InitializeSecurityContext(credential = System.Net.SafeFreeCredential_SECURITY, context = 14d12d72220:1f8df7d55a0, targetName = <url>, inFlags = ReplayDetect, SequenceDetect, Confidentiality, AllocateMemory, InitManualCredValidation)
DateTime=2022-11-24T08:53:41.6684073Z
System.Net Information: 0 : [5000] InitializeSecurityContext(Anzahl von In-Buffers = 2, Länge von Out-Buffer = 0, zurückgegebener Code = IllegalMessage).
DateTime=2022-11-24T08:53:41.6684073Z
System.Net.Sockets Verbose: 0 : [5000] Entering Socket#51781231::Dispose()
DateTime=2022-11-24T08:53:41.6864476Z
System.Net Error: 0 : [5000] Ausnahme in HttpWebRequest#54443429:: - Die Anfrage wurde abgebrochen: Es konnte kein geschützter SSL/TLS-Kanal erstellt werden...
DateTime=2022-11-24T08:53:41.6914164Z
System.Net Verbose: 0 : [5000] Entering HttpWebRequest#54443429::EndGetResponse()
DateTime=2022-11-24T08:53:41.6934164Z
System.Net Error: 0 : [5000] Ausnahme in HttpWebRequest#54443429::EndGetResponse - Die Anfrage wurde abgebrochen: Es konnte kein geschützter SSL/TLS-Kanal erstellt werden...
DateTime=2022-11-24T08:53:41.6934164Z
System.Net Information: 0 : [8740] ServicePoint#58366981::CloseConnectionGroupInternal(62809978)
DateTime=2022-11-24T08:53:41.7084210Z
System.Net Information: 0 : [8740] ServicePoint#58366981::CloseConnectionGroupHelper(connectionGroupName=62809978, closeInternal=True)
DateTime=2022-11-24T08:53:41.7094117Z
System.Net Information: 0 : [8740] ServicePoint#58366981::ReleaseConnectionGroup(62809978S>I>)
DateTime=2022-11-24T08:53:41.7104119Z
System.Net Information: 0 : [8740] ServicePoint#58366981::ReleaseConnectionGroup, returning(true)
DateTime=2022-11-24T08:53:41.7114113Z
System.Net Information: 0 : [8740] ServicePoint#58366981::CloseConnectionGroupHelper, returning(True)
DateTime=2022-11-24T08:53:41.7114113Z
According to this page (https://learn.microsoft.com/en-us/dotnet/framework/migration-guide/how-to-determine-which-versions-are-installed) and the powershell query, the .Net framework installed is 4.8+:
> (Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full").Release
528049
According to https://www.ssllabs.com/ssltest/ the requested page uses TLS 1.2
What do i have to do to get a working connection.