exception error on server iis input string not in correct format

I work on asp.net MVC application . I get error input string not in correct format

i don’t know what is the reason of this issue and which line give me this issue

on local pc issue not happen but on iis publish site web app it happen

so how i know reason of this issue please

and can i do some enhancement on code to display more clear message or prevent this error from happen

And which line generate issue

if i write some parsing on code with wrong way please tell me to correct it

my action code as below

public class ResignationRequester
  {
  [Display(Name = "Employee No : ")]   [Required,]   
public int EmpID { get; set; }
[Display(Name = "Line Manager: ")]
public int? LineManager { get; set; }
  }

[HttpPost]
  public JsonResult RequesterIndex(ResignationRequester resignationRequester)
  {
      dynamic responseData = new ExpandoObject();
      responseData.success = false;
      responseData.message = "";
      try
      {
          var filenumber = resignationRequester.EmpID;
          if (Session[SessionKeys.UserCode] != null)
          {
              JDEUtility jde = new JDEUtility();
       
              resignationRequester.EmpID = Convert.ToInt32(Session[SessionKeys.UserCode]);
                        
           
             


              if (ModelState.IsValid)
              {

                  
                 
                 
                  if (Convert.ToString(resignationRequester.LineManager).Length < 6 && !string.IsNullOrEmpty(resignationRequester.LineManager.ToString()))
                  {
                      responseData.success = false;
                      responseData.message = "Length Line Manager Must Be equal 6 or More";
                      return Json(responseData);

                  }
                  
                  if (Convert.ToInt32(resignationRequester.EmpID) == Convert.ToInt32(resignationRequester.DirectManager) &&
  Convert.ToInt32(resignationRequester.EmpID) == Convert.ToInt32(resignationRequester.LineManager)
  && !string.IsNullOrEmpty(Convert.ToString(resignationRequester.LineManager)) && !string.IsNullOrEmpty(Convert.ToString(resignationRequester.DirectManager)))
                  {
                      responseData.success = false;
                      responseData.message = "Requester Have Same Number of Manager";
                      return Json(responseData);

                  }
                  if (!string.IsNullOrEmpty(Convert.ToString(resignationRequester.LineManager)))
                  {
                      responseData.success = false;
                      responseData.message = "Line Manager Name Blank";
                      return Json(responseData);

                  }
                 

                 
                 



                 
               
                  if (string.IsNullOrEmpty(ViewBag.errorMsg))
                  {
                      responseData.success = true;
                      responseData.message = "Resignation Submission form Created successfully";
                      

                      return Json(responseData);
                  }



              }
              
          }
         
      }
     
      catch (System.FormatException ex)
      {
          responseData.success = false;
          responseData.message = ex.Message;
      }
      return Json(responseData);
      
  }

on UI View this is what i do

$("#txtLineManagerId").autocomplete({
            source: function (request, response) {
                var searchText = $("#txtLineManagerId").val();
                console.log("search text" + searchText)
                $.ajax({
                    url: '@Url.Action("GetAllEmployeeBasedSearchText", "Resignation")',
                    data: { searchText: searchText },
                    method: "GET",
                    dataType: "json",
                    success: function (data) {
                        if (!data.length) {
                            var result = [
                                {
                                    label: 'No matches found',
                                    value: response.term
                                }
                            ];
                            response(result);
                        }
                        else {
                           
                            response($.map(data, function (item) {
                                
                                return {
                                    
                                    label: "File No : " + item.EmployeeID + " - " + "Name :" + item.EmployeeName + " - " +
                                        "Designation : " + item.Designation, value: item.EmployeeID,
                                    employeeName: item.EmployeeName,
                                    designation: item.Designation
                                };

                            }))
                        }
                    }
                });
            },
            position: { my: "right top", at: "right bottom" },
            appendTo: '#searchContainer',
            select: function (event, ui) {
              
                $("#LineManagerName").val(ui.item.employeeName);
             
            },
            minLength: 2,
          
        }).data("ui-autocomplete")._renderItem = function (ul, item) {
            console.log("after autocomplete" + item);
            
            return $("<li>")
                .append("<div>File No: " + item.value + "<br/>Name: " + item.employeeName + "<br/>Designation: " + item.designation + "</div>")
                .appendTo(ul);
            
        };

 <div class="form-group col-md-6">
     <div class="col-md-5">
         @Html.LabelFor(model => model.LineManager, htmlAttributes: new { @class = "control-label" })


     </div>

     <div class="col-md-7">
         @Html.EditorFor(model => model.LineManager, new
         {
             htmlAttributes = new
             {
                 @class = "form-control"
        ,
                 id = "txtLineManagerId"
             }
         })
         <div id="searchContainer">

         </div>
 
        
     </div>
 </div>