I have an asp net core application developed with .NET 5.

In this application, there is a controller (ACSController) that manages responses from an Identity Provider. The controller is the follow:

public class ACSController : Controller
{
    private readonly IRequestRepository _requestRepository;
    private readonly IHttpClientFactory _httpClientFactory;
    private readonly IIdpRepository _idpRepository;
    private readonly IResponseRepository _responseRepository;

    private readonly AppSessionViewModel _session;

    public ACSController(IRequestRepository requestRepository, IHttpClientFactory httpClientFactory,
        IIdpRepository idpRepository, IResponseRepository responseRepository, AppSessionViewModel session)
    {
        _requestRepository = requestRepository;
        _httpClientFactory = httpClientFactory;
        _idpRepository = idpRepository;
        _responseRepository = responseRepository;

        _session = session;
    }

    [HttpPost]
    public async Task<IActionResult> IndexAsync(IFormCollection form)
    {
        var base64Response = form["SAMLResponse"].ToString();
        var response = SAMLHelper.GetAuthnResponse(base64Response);

        var cachedRequest = _requestRepository.Read();
        var idpMetadata = await SamlHandler.DownloadIdPMetadata(_httpClientFactory,
            _idpRepository.Read().OrganizationUrlMetadata);

        var validationResult = ResponseValidator.ValidateAuthnResponse(response, cachedRequest, idpMetadata);

        if (validationResult.IsSuccess)
        {
            _responseRepository.Write(response);
            _session.Logged = true;
            ViewData["UserInfo"] = CreateUserInfo(response);
            return View();
        }
        else
        {
            ViewData["Message"] = validationResult.Error;
            return View("Error");
        }
    }

    [HttpPost]
    public IActionResult Logout(IFormCollection form)
    {
        var base64Response = form["SAMLResponse"].ToString();
        return View();
    }

    

    [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
    public IActionResult Error()
    {
        return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
    }

    #region Utilities

    private Dictionary<string, string> CreateUserInfo(AuthnResponse response)
    {
        if (response == null) throw new ArgumentException(nameof(response));

        var attributes = response.GetAssertion().GetAttributeStatement().Items;

        var userDictionary = new Dictionary<string, string>();

        foreach (var attribute in attributes)
        {
            var attr = (AttributeType)attribute;
            userDictionary.Add(attr.Name, (string)attr.AttributeValue.First());
        }

        return userDictionary;
    }

    #endregion
}

In my case, there is no problem when acs/index method is called by a POST request.

When I invoke the Logout procedure (from another controller) the request is sent correctly by my application but when IdP responds to me I obtain this response on my browser:

Update：

If you want to decode a base64string,try to use:

byte[] data = Convert.FromBase64String(SAMLResponse);
 string decodedString = Encoding.UTF8.GetString(data);