I've been getting this error of lately on Unity 2018.4.30f1, looked on the internet, and didn't found any real solution to this.
PlayFab EditorExtensions: Caught an error:Invalid JSON string
UnityEngine.Debug:LogError(Object)
PlayFab.PfEditor.PlayFabEditor:StateUpdateHandler(EdExStates, String, String) (at Assets/PlayFabEditorExtensions/Editor/PlayFabEditor.cs:332)
PlayFab.PfEditor.PlayFabEditor:RaiseStateUpdate(EdExStates, String, String) (at Assets/PlayFabEditorExtensions/Editor/PlayFabEditor.cs:247)
PlayFab.PfEditor.PlayFabEditorHelper:SharedErrorCallback(PlayFabError) (at Assets/PlayFabEditorExtensions/Editor/Scripts/Utils/PlayFabEditorHelper.cs:127)
PlayFab.PfEditor.<>c:<OnLoginButtonClicked>b__14_1(PlayFabError) (at Assets/PlayFabEditorExtensions/Editor/Scripts/Panels/PlayFabEditorAuthenticate.cs:299)
PlayFab.PfEditor.PlayFabEditorHttp:OnWwwError(Action`1, String) (at Assets/PlayFabEditorExtensions/Editor/Scripts/PlayFabEditorSDK/PlayFabEditorHttp.cs:130)
PlayFab.PfEditor.<>c__DisplayClass2_0`2:<MakeApiCall>b__1(String) (at Assets/PlayFabEditorExtensions/Editor/Scripts/PlayFabEditorSDK/PlayFabEditorHttp.cs:95)
PlayFab.PfEditor.<Post>d__7:MoveNext() (at Assets/PlayFabEditorExtensions/Editor/Scripts/PlayFabEditorSDK/PlayFabEditorHttp.cs:177)
PlayFab.PfEditor.EditorCoroutine:Update() (at Assets/PlayFabEditorExtensions/Editor/Scripts/Utils/EditorCoroutine.cs:103)
UnityEditor.EditorApplication:Internal_CallUpdateFunctions() (at C:/buildslave/unity/build/Editor/Mono/EditorApplication.cs:200)