redirect to return url after login

I have a link in my razor view like this:

 <a href="Home/Login?ReturnUrl=Disputes/Index"> disputes</a>

Inside my login's action method, I am using this:

 public ActionResult Login(string returnUrl) {
   if (string.IsNullOrEmpty(returnUrl) && Request.UrlReferrer != null)
         returnUrl = Server.UrlEncode(Request.UrlReferrer.PathAndQuery);

   if (Url.IsLocalUrl(returnUrl) && !string.IsNullOrEmpty(returnUrl))
   {
      ViewBag.ReturnURL = returnUrl;
   }

   return View();
 }

In view I am using this:

 @Html.Hidden("returnUrl",@Request.QueryString)

Then in post action method:

 public ActionResult LogOn(LogOnModel model, string returnUrl)
 {
   if (ModelState.IsValid)
   {
      if (membershipService.ValidateUser(model.UserName, model.Password, model.Type))
      {
         formsAuthenticationService.SignIn(model.UserName, model.RememberMe);
         SetUserInfo(model.UserName);

         string decodedUrl = "";
         if (!string.IsNullOrEmpty(returnUrl))
            decodedUrl = Server.UrlDecode(returnUrl);

         if (Url.IsLocalUrl(decodedUrl))                    
            return Redirect(decodedUrl);
         else
            return Redirect("Home", Index);

      }
   }
 }

It is redirecting to:/Disputes/Index but it should go to myApp/Disputes/Index where the url with query string is like this. /myApp/Home/Login?ReturnUrl=/Disputes/Index

How can I solve this issue?