На этот вопрос уже есть ответ здесь:

Free jqgrid использует настройки

autoencode: true

Определение сетки содержит:

                $grid.jqGrid({
                    url: '/admin/API/Entity',
                    datatype: "json",
                    editurl:'/admin/Detail/Edit'

Встроенное редактирование<a символ вводится в столбец Nimetus, а данные отправляются на сервер в формате URL.

Request URL:http://localhost:52216/admin/Detail/Edit?_entity=DokG&_dokumnr=135322&_vmnr=0
Request Method:POST
Status Code:490 OK
Response Headers
view source
Cache-Control:private, s-maxage=0
Content-Length:122
Content-Type:application/json; charset=utf-8
Date:Mon, 23 Nov 2015 15:31:54 GMT
Server:Microsoft-IIS/10.0
X-AspNet-Version:4.0.30319
X-SourceFiles:=?UTF-8?B?STpccmFhbWF0XEVldmFXZWJcRWV2YS5FcnBcRGV0YWlsXEVkaXQ=?=
Request Headers

POST /admin/Detail/Edit?_entity=DokG&_dokumnr=135322&_vmnr=0 HTTP/1.1
Host: localhost:52216
Connection: keep-alive
Content-Length: 1724
Accept: */*
Origin: http://localhost:52216
X-Requested-With: XMLHttpRequest

Query String Parameters

_entity:DokG
_dokumnr:135322
_vmnr:0
Form Data
view parsed
Kogus=&Nimetus=%3Ca&Mootyhik0_nimetus=&Hinnak=&Hind=&Myygikood=&_rowsum=0.00&Rtellimus=&Toode=&Kulukonto=&Yhik=&Id=0&Dokumnr=135322&Reanr=3&_oper=edit&_rowid=1648&_dokdata=%5B%7B%22name%22%3A%22Klient0_nimi%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Tasudok%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Kuupaev%22%2C%22value%22%3A%222015-11-23%22%7D%2C%7B%22name%22%3A%22Kellaaeg%22%2C%22value%22%3A%2217+29%22%7D%2C%7B%22name%22%3A%22Maksetin1_tingimus%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Tarnekla2_nimetus%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Konto3_tekst%22%2C%22value%22%3A%22112%22%7D%2C%7B%22name%22%3A%22Yksus%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Tasukuup%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Knr%22%2C%22value%22%3A%2213111%22%7D%2C%7B%22name%22%3A%22Alamdok4_nimetus%22%2C%22value%22%3A%22ASL%22%7D%2C%7B%22name%22%3A%22Raha%22%2C%22value%22%3A%22EUR%22%7D%2C%7B%22name%22%3A%22Eimuuda%22%2C%22value%22%3A%22false%22%7D%2C%7B%22name%22%3A%22Prladu5_laonimi%22%2C%22value%22%3A%221%22%7D%2C%7B%22name%22%3A%22Krdokumnr%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Tekst1%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Pais7obj%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Klient%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Maksetin%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Tarneklaus%22%2C%22value%22%3A%22%22%7D%2C%7B%22name%22%3A%22Arvekonto%22%2C%22value%22%3A%22112%22%7D%2C%7B%22name%22%3A%22Oper%22%2C%22value%22%3A%22ASL%22%7D%2C%7B%22name%22%3A%22Laonr%22%2C%22value%22%3A%221%22%7D%2C%7B%22name%22%3A%22Dokumnr%22%2C%22value%22%3A%22135322%22%7D%2C%7B%22name%22%3A%22Kinnitatud%22%2C%22value%22%3A%22False%22%7D%5D

Контроллер ASP.NET MVC4 использует отражение для вызова метода, который читает это значение, используяControllerContext.HttpContext.Request.Form["Nimetus"]

Этот доступ вызывает исключение

System.Web.HttpRequestValidationException was unhandled by user code

  Message=A potentially dangerous Request.Form value was detected from the client (Nimetus="<a").

Исключение происходит в строке

ControllerContext.HttpContext.Request.Form["Nimetus"]

Как решить проблему?

Изменить сигнатуру метода контроллера

[AcceptVerbs(HttpVerbs.Post)]
[HandleJsonException]
public JsonResult Edit(string _entity, string _dokdata, int? _dokumnr, string _rowid,
    int? _vmnr, string _isik)

Обновить

Данные заголовка документа передаются как параметр _dokdata с помощью

    extraparam: { _dokdata: getEevaFormData },

в

   $.extend(true,$.jgrid.inlineEdit, {
        position: "beforeSelected",
        focusField: false,
        restoreAfterError: false,
        afterrestorefunc: function(rowId) {
            updateButtonState($grid, rowId);
            setFocusToGrid();
            lastSelectedRow = undefined;
        },
        aftersavefunc: function(rowId, response) {

            afterSaveFuncAfterAdd.call(this, rowId, response);

        },

        oneditfunc: function(rowId) {
            onInlineEdit(rowId);
            updateButtonState($grid, rowId);
        },
        keys: true,
        rowID: '_empty',
        useDefValues: true,
        extraparam: { _dokdata: getEevaFormData },
        errorfunc: errorfunc
    });

Параметр _dokdata контроллера теперь содержит определение функции с заменой на"

function getEevaFormData() {
    return JSON.stringify($("#_form").serializeArray());
}

На самом деле он должен содержать строку json, которая является результатом вызова этой функции