conn_7 Error control message: ClientId[6528662]:ErrorId[8]: Error on incoming message

I am currently creating a firebase realtime database transaction in unity c# and keep getting this error:

[Error log]
[conn_7] Error control message: ClientId[6528662]:ErrorId[8]: Error on incoming message
UnityEngine.Debug:LogError (object)
Firebase.Platform.FirebaseLogger:LogMessage (Firebase.Platform.PlatformLogLevel,string) (at /Users/runner/work/firebase-unity-sdk/firebase-unity-sdk/app/platform/Unity/FirebaseLogger.cs:95)
Firebase.LogUtil:LogMessage (Firebase.LogLevel,string) (at /Users/runner/work/firebase-unity-sdk/firebase-unity-sdk/app/src/LogUtil.cs:69)
Firebase.LogUtil:LogMessageFromCallback (Firebase.LogLevel,string) (at /Users/runner/work/firebase-unity-sdk/firebase-unity-sdk/app/src/LogUtil.cs:77)
Firebase.AppUtil:PollCallbacks () (at /Users/runner/work/firebase-unity-sdk/firebase-unity-sdk/android_unity/armeabi-v7a/app/swig/Firebase.App_fixed.cs:4604)
Firebase.Platform.FirebaseAppUtils:PollCallbacks () (at /Users/runner/work/firebase-unity-sdk/firebase-unity-sdk/app/platform/FirebaseAppUtils.cs:33)
Firebase.Platform.FirebaseHandler:Update () (at /Users/runner/work/firebase-unity-sdk/firebase-unity-sdk/app/platform/Unity/FirebaseHandler.cs:208)
Firebase.Platform.FirebaseMonoBehaviour:Update () (at /Users/runner/work/firebase-unity-sdk/firebase-unity-sdk/app/platform/Unity/FirebaseMonoBehaviour.cs:45)

Source Code:

public void SendjointPoints()
{
    if (!TransactionComplete)
    {
        Debug.Log("not finished Transaction.  skip a frame.");
        return;
    }

    TransactionComplete = false;
    DatabaseReference reference = FirebaseDatabase.DefaultInstance.GetReference("jointPoints");
    Debug.Log("Running Transaction...");
    reference.RunTransaction(SendTransaction)
      .ContinueWithOnMainThread(task => {
          if (task.Exception != null)
          {
              TransactionComplete = true;
              Debug.Log(task.Exception.ToString());
          }
          else if (task.IsCompleted)
          {
              TransactionComplete = true;
              Debug.Log("Transaction complete.");
          }
      });
}

This error freezes Unity. any more transaction is not working until Unity Stop. My code wants 1~2 transaction per a second.

Any idea why this is happening?

My code is trying to navigate through the various levels of mutabledata I got from my database reference.

• Unity version 2021.3.5f1
• Building for Android and Window
• IDE: Microsoft Visual Studio
• firebase_unity_sdk_9.4.0 (dotnet4 : FirebaseDatabase.unitypackage)