Rails error: "param is missing or the value is empty" suddenly after remove gem

Question

When i create a task an ajax function is called to send a specific params data. It was working very well. Then i've installed "wicked_pdf" and "wkhtmltopdf-binary". After learning that on windows I have problems with this tools, i removed them by using "gem uninstall wicked_pdf/wkhtmltopdf-binary". (I removed manually all wicked_pdf config file). After this when i create a new task I have this error on my rails application (windows): " param is missing or the value is empty: task". If I delete the ajax code it works fine but not as before.

Ajax (was working very well):

$( "#new_task" ).submit(function(event) {
event.preventDefault();
var value = $('#task_value').val().replace(assignee,'');
value.replace(/ /g,'');
$('#new_task').find('input[name="task[value]"]').val().replace(assignee,'');

 $.ajax({
   url: "/tasks",
   type: "post",
   contentType: "application/json",

   data: JSON.stringify({ assignee: assignee, value: value }),
   success: function(data) {
       alert('successfully');
   },
   error: function(xhr, textStatus, error) {
     alert(xhr.statusText+" "+textStatus+" "+error);
   }
});

});  

controller (it was working, i never changed it):

def create
    @task = Task.new(task_params)
    @task.user = current_user
    set_date
    set_category
    respond_to do |format|
      if @task.save
        format.html { redirect_to tasks_url, notice: 'Task was successfully created. '+task_params.inspect}

        #format.html { redirect_to @task, notice: 'Task was successfully created.' }
        format.json { render :show, status: :created, location: @task }
      else
        format.html { render :new }
        format.json { render json: @task.errors, status: :unprocessable_entity }
      end
    end
  end

And then in the index view i have this partial <%= render 'form', task: @task %> to a standard form (value and submit, with create action). What's the problem? I messed up with wicked_pdf?