When model is not valid, return to partial view inside a view, with error message

From what I can see you are using a standard Html.BeginForm POSTing to the ChangeEvaluator controller action which either performs a redirect or returns a partial view if validation fails.

So the behavior you are observing is perfectly normal. You will have to submit this form using AJAX if you want to achieve that:

@using (Ajax.BeginForm("ChangeEvaluator", "Ontwikkelplan", new AjaxOptions { OnSuccess = "handleSuccess" })){    ...}

and then you could adapt your controller action so that in case of success it doesn't redirect but it returns a Json object containing the url to redirect to:

[HttpPost]public ActionResult ChangeEvaluator(EvaluatorWijzigenOPViewModel ewopvm){    if (ModelState.IsValid)    {        ...        return Json(new { redirectTo = Url.Action("Evaluatorenlijst") });    }    return PartialView("EvaluatorWijzigenPartial", ewopvm);}

and finally write the handleSuccess javascript function:

function handleSuccess(result) {    if (result.redirectTo) {        // The controller action returned a JSON object with the redirectTo property        // let's redirect to this location:        window.location.href = result.redirectTo;    } else {        // The controller action returned a partial view with the form and the errors        // So we need to update some containing DIV with it:        $('#someDivThatCOntainsYourForm').html(result);    }}