How to handle a JSON input error with HttpClient PostAsync which does not contain any JSON tokens

Question

I have the following 2 routines:

public async Task<Token> GetToken(string username, string password)
{
    var transurl = $"{burl}users/auth/";
    using (var c = new HttpClient())
    {
        var cs = $"{{  \"username\": \"{username}\", \"password\": \"{password}\"}}";
        var token = await PostDataAsync<Token>(c, transurl, cs);
        return token;
    }
}

public async Task<T> PostDataAsync<T>(HttpClient c, string url, string content)
{
    var sc = new StringContent(content, Encoding.UTF8, "application/json");
    var res = await c.PostAsync(url, sc); //this line seems to produce the error
    if (res.IsSuccessStatusCode)
    {
        return await res.Content.ReadFromJsonAsync<T>();
    }
    else
    {
        var errs = await res.Content.ReadAsStringAsync();
        var nordExc = JsonSerializer.Deserialize<Exception>(errs);
        var nexc2 = new Exception(errs);
        throw nexc2;
    }
}

The error which I have got is

The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0.

And as far as I can see it is the codeline await c.PostAsync(url, sc); which reports this error. The value of the variable content of the PostDataAsync routine in my case shows the string "{ "username": "myUsername", "password": "myPassword"}" which is a valid JSON format. I have checked the evaluation of this value in Visual Studio - and selected JSON Visualizer, and the content of the variable is fine. And because of the error the value of IsSuccessStatusCode is false.

How to handle this error?