My Problem is that, the result from the controller replaces the entire view with its content, instead of just updating an element in the view with its JSON.

I tried to change data type in AJAX-Post, but it didn’t work

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Index(DTO_SomeObject surveyData)
{
...
    if (!ModelState.IsValid)
    {
        //var errors = ModelState.Values.SelectMany(v => v.Errors).Select(e => e.ErrorMessage).ToList();
        //return Json(new { success = false, errors = string.Join("<br>", errors) });

        var errors = ModelState.Values.SelectMany(v => v.Errors)
                               .Select(e => e.ErrorMessage)
                               .Aggregate((current, next) => current + "<br>" + next);
        return Json(new { success = false, errors });
    }
}

Here is the frontend with it’s HTML and razor content:

    @model SurveyRetirementAspNet.Models.DTO_SomeObject
    @* @model Datalayer.Models.Global.Classes.DTO_SomeObject *@
    
    @{
        ViewData["Title"] = "Umfrage: " + @Model.Name;
    }
    
    <h1 class="text-center mt-4 mb-5"><u style="font-weight:bolder">@Model.Name</u></h1>
    <div class="container">
        @* <form asp-action="Index" method="post" class="card card-body shadow-lg p-5 border-0"> *@
        <form id="surveyForm" asp-action="Index" method="post" class="card card-body shadow-lg p-5 border-0">
           ...
            <div class="separator"></div>
            <div id="errorMessages"></div>
            <button type="submit" class="btn btn-primary w-100">Umfrage absenden</button>
        </form>
    </div>

<script type="text/javascript">
 
    $(document).ready(function () {
        $('#surveyForm').on('submit', function (e) {
            e.preventDefault();  // Prevent normal form submission

            var formData = $(this).serialize();  // Serialize the form data

            $.ajax({
                type: 'POST',
                url: $(this).attr('action'),  // Use the action from the form
                data: formData,
                dataType: 'json',  // Expect JSON response from the server
                success: function (response) {
                    if (response.success) {
                        window.location.href = '/path/to/success/page';  // Redirect on success
                    } else {
                        // Display error messages returned from the server
                        // $('#errorMessages').html(response.errors);
                        console.log("Ajax error");  // Fehler anzeigen, wenn Ajax fehlschlägt

                    }
                },
                error: function (jqXHR, textStatus, errorThrown) {
                    $('#errorMessages').html('Error: ' + textStatus + ' - ' + errorThrown);
                }
            });
        });
    });


  
</script>

How should it be right?