Devise/Cancan Signout User On Controller Action

I have implemented Devise for Authentication and Authorization in ROR
application everything seems fine but getting one issue. I have two
modals "Account" and "Transactiona" , and so two controllers
respectively.

My Transaction Index view call one of Account Controller method like
this

$.post("accounts/our_miles_balance/?account_number="+$("#account_number").val(),function(data)
{
   $("#our_miles_balance").val(data);
});

When this ajax post run it gives following error and sign out admin user

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Here is my Ability Class

class Ability
include CanCan::Ability

def initialize(user)
  user ||= User.new # guest user
  if user.role == 1 #admin
    can :manage, :all
    can :read, :all
  elsif user.role == 2 #Vendor
    can :manage, VendorTransaction
    can :index, Account
  end
end
end

Ok Here is my Transaction controller

require 'csv'
class TransactionsController < ApplicationController
load_and_authorize_resource
helper_method :sort_column, :sort_direction

respond_to :html, :js
def index
  per_page = 40
  @transactions = Transaction.search(params[:id]).order(sort_column + "
" + sort_direction)

respond_to do |format|
  format.html # index.html.erb
  format.csv { render :csv => @transactions}
end

AND Account Controller

class AccountsController < ApplicationController
load_and_authorize_resource
helper_method :sort_column, :sort_direction
def index
   @accounts = Account.search(params[:program_id]
  respond_to do |format|
    format.html # index.html.erb
    format.json { render :json => @accounts}
end

def our_miles_balance
   a = Account.find_by_account_number(params[:account_number])
   @miles = Account.our_miles_balance(a.id) if ?a!=nil
    respond_to do |format|
     format.json { render json: @miles}
  end
end
end

What i m doing wrong here, please help....