For some unknown reasons, when I tried to log into WordPress to edit or add posts, the screens came up with “Unknown error (invalid-json). Please contact an administrator for more info.”. I tried my test blog without the BWP reCaptcha and I could log in without any problem. So I renamed the plugin and voila, the error message is gone and I can log in without any problems. This problem also exist in the comment section, we also need to remove the option for the comment if we want to use this.