How to process received JSON in unity - knocks out an error?

Question

JSON

Guys, help out the stupid, I ran into a problem that when I send a post request to the server, the server generates a key and sends it to JSON, however, no matter how much I googled, I get an error.
I launch a coroutine

PostStruct user = new PostStruct()
        {
            Login = "test",
            Password = "test",
            key = "TestKey" // исходящий ключ по дефолту в принципе пустой
        };
 
        string json = JsonUtility.ToJson(user);
 
        //Debug.Log(json);
 
        UnityWebRequest request = UnityWebRequest.Post("https://" + serverURI + "/login.php", json);
 
        byte[] postBytes = Encoding.UTF8.GetBytes(json);
 
        UploadHandler uploadHandler = new UploadHandlerRaw(postBytes);
 
        request.uploadHandler = uploadHandler;
 
        request.SetRequestHeader("Content-Type", "application/json; charset=UTF-8");
 
        yield return request.SendWebRequest();
 
        Debug.Log(request.downloadHandler.text);
 
        PostStruct postStructFromServer = JsonUtility.FromJson<PostStruct>(request.downloadHandler.text);

It seems to be sent and sent, but on the last line, where the parsing is going on, it knocks out
the Error Code

ArgumentException: JSON parse error: Invalid value.
UnityEngine.JsonUtility.FromJson (System.String json, System.Type type) (at <bd9566cca22541e58ad28d1fa2849830>:0)
UnityEngine.JsonUtility.FromJson[T] (System.String json) (at <bd9566cca22541e58ad28d1fa2849830>:0)
AuthRequest+<SendRequest>d__3.MoveNext () (at Assets/Auth/Scripts/AuthRequest.cs:58)
UnityEngine.SetupCoroutine.InvokeMoveNext (System.Collections.IEnumerator enumerator, System.IntPtr returnValueAddress) (at <04258d1cdc1044248c2a17a6a31a3cf7>:0)

But in the debug log it shows that the answer came, that it does not parse, please help

Array
(
    [Login] => test
    [Password] => test
    [key] => TestKey
)
{"Login":"test","Password":"test","key":"kKCsDu836n"}
UnityEngine.Debug:Log(Object)
<SendRequest>d__3:MoveNext() (at Assets/Auth/Scripts/AuthRequest.cs:56)
UnityEngine.SetupCoroutine:InvokeMoveNext(IEnumerator, IntPtr)

ps tried to google, played with the structure