The remote validation areaName is not working properly. Please check the generated data-val-remote-url and you can find the current area in the value.
[System.Web.Mvc.Remote("IsUserNameAvailable", "Account", "", ErrorMessageResourceName = "RemoteUserName", ErrorMessageResourceType = typeof(Resources.Models.ErrorMessages))]
[System.Web.Mvc.Remote("IsUserNameAvailable", "Account", "", ErrorMessageResourceName = "RemoteUserName", ErrorMessageResourceType = typeof(Resources.Models.ErrorMessages))]