Corrected with rubocop cop 'Style/ClassVars'.

.rubocop.yml

@@ -187,7 +187,5 @@ Style/Documentation:
 
 Lint/RescueException:
   Enabled: false
-Style/ClassVars:
-  Enabled: false
 Lint/UselessAssignment:
   Enabled: false

app/models/application_model.rb

@@ -41,12 +41,13 @@ class ApplicationModel < ActiveRecord::Base
 
   attr_accessor :history_changes_last_done
 
-  @@import_class_list = ['Ticket', 'Ticket::Article', 'History', 'Ticket::State', 'Ticket::StateType', 'Ticket::Priority', 'Group', 'User', 'Role' ]
-
   def check_attributes_protected
 
+    import_class_list = ['Ticket', 'Ticket::Article', 'History', 'Ticket::State', 'Ticket::StateType', 'Ticket::Priority', 'Group', 'User', 'Role' ]
+
     # do noting, use id as it is
-    return if !Setting.get('system_init_done') || ( Setting.get('import_mode') && @@import_class_list.include?( self.class.to_s ) )
+    return if !Setting.get('system_init_done')
+    return if Setting.get('import_mode') && import_class_list.include?( self.class.to_s )
 
     self[:id] = nil
   end

app/models/history.rb

@@ -11,9 +11,11 @@ class History < ApplicationModel
   #  before_validation :check_type, :check_object
   #  attr_writer :history_type, :history_object
 
+  # rubocop:disable Style/ClassVars
   @@cache_type = {}
   @@cache_object = {}
   @@cache_attribute = {}
+# rubocop:enable Style/ClassVars
 
 =begin
 

app/models/object_lookup.rb

@@ -1,7 +1,7 @@
 # Copyright (C) 2012-2014 Zammad Foundation, http://zammad-foundation.org/
 
 class ObjectLookup < ApplicationModel
-  @@cache_object = {}
+  @@cache_object = {} # rubocop:disable Style/ClassVars
 
   def self.by_id( id )
 

app/models/package.rb

@@ -2,7 +2,7 @@
 
 require 'rexml/document'
 class Package < ApplicationModel
-  @@root = Rails.root.to_s
+  @@root = Rails.root.to_s # rubocop:disable Style/ClassVars
 
   # build package based on .szpm
   # Package.build(
@@ -437,7 +437,7 @@ class Package < ApplicationModel
   end
 
   class Migration < ApplicationModel
-    @@root = Rails.root.to_s
+    @@root = Rails.root.to_s # rubocop:disable Style/ClassVars
 
     def self.migrate( package, direction = 'normal' )
       location = @@root + '/db/addon/' + package.underscore

app/models/setting.rb

@@ -9,7 +9,7 @@ class Setting < ApplicationModel
   after_create  :delete_cache
   after_update  :delete_cache
 
-  @@current = {}
+  @@current = {} # rubocop:disable Style/ClassVars
 
   def self.load
 

app/models/tag.rb

@@ -4,8 +4,10 @@ class Tag < ApplicationModel
   belongs_to :tag_object,       class_name: 'Tag::Object'
   belongs_to :tag_item,         class_name: 'Tag::Item'
 
+  # rubocop:disable Style/ClassVars
   @@cache_item = {}
   @@cache_object = {}
+  # rubocop:enable Style/ClassVars
 
   def self.tag_add(data)
 

app/models/type_lookup.rb

@@ -1,7 +1,7 @@
 # Copyright (C) 2012-2014 Zammad Foundation, http://zammad-foundation.org/
 
 class TypeLookup < ApplicationModel
-  @@cache_object = {}
+  @@cache_object = {} # rubocop:disable Style/ClassVars
 
   def self.by_id( id )
 

lib/sessions.rb

@@ -13,7 +13,7 @@ module Sessions
   @path = "#{@root}/tmp/websocket"
 
   # create global vars for threads
-  @@client_threads = {}
+  @@client_threads = {} # rubocop:disable Style/ClassVars
 
 =begin
 

lib/sessions/cache_in.rb

@@ -1,8 +1,11 @@
 module Sessions::CacheIn
+
+  # rubocop:disable Style/ClassVars
   @@data = {}
   @@data_time = {}
   @@expires_in = {}
   @@expires_in_ttl = {}
+  # rubocop:enable Style/ClassVars
 
   def self.delete( key )
     @@data.delete( key )