Problem with uploading file

[Unkind]

i have method that upload xls file<br>

on 1 computer it's all ok, but on another i have error<br>

Uncaught You're trying to decode an invalid JSON String: <div id="header"><h1>Server Error</h1></div>
<div id="content">
 <div class="content-container"><fieldset>
  <h2>500 - Internal server error.</h2>
  <h3>There is a problem with the resource you are looking for, and it cannot be displayed.</h3>
 </fieldset></div>
</div>

what i do not?

p.s. i have another method? that upload images, this method work on all computers.
realization like method for xls

         if (param == 'addFile') {            var itemRs = this.getGrid().getSelectionModel().getSelection();
            var Sd = itemRs[0];
            var itemId = Sd.get('Id');
            if (form.isValid()) {
                form.submit({
                    url: '/ServiceDemand/ServiceDemands/Upload',
                    params: {
                        itemId: itemId
                    },
                    waitMsg: TR.Config.WaitMsg,
                    success: function (form, o) {
                        win.close();
                        TR.Globals.showInfo(o.result.msg);
                        me.doRefreshFiles();
                    },
                    failure: function (form, o) {
                        TR.Globals.showError(o.result.msg);
                    }
                });
            }
        }else if (param == 'import') {
            if (form.isValid()) {
                form.submit({
                    url: '/ServiceDemand/ServiceDemands/Import',
                    waitMsg: TR.Config.WaitMsg,
                    success: function (form, o) {
                        win.close();
                        console.log(o.result.result);
                        TR.Globals.showInfo(o.result.result);
                        me.getGrid().store.load();
                        
                    }
                    ,failure: function (form, o) {
                        TR.Globals.showError(o.result.result);
                    }
                });
            }
        }

from controller i return

public ActionResult Upload()        {
           
            return Content("{success:true, result:\"Файл успешно добавлен.\"}");
        }

[mitchellsimoens]

The form expects JSON in return, you are returning HTML.