Import errors into django-cms

I am new to django and after going through some tutorial django

I started using django-cms

(I had a requirement to develop a cms site with django-cms).

I edited the settings file and added all the middle programs, applications that are needed for cms

, so everything goes well and started creating html templates, but sometimes after restarting the server and getting below weird errors

Error_1

Django Version: 1.4.3
Exception Type: ImportError
Exception Value:    
cannot import name plugin_pool
Exception Location: /usr/lib/python2.7/site-packages/cms/admin/__init__.py in <module>, line 7
Python Executable:  /usr/bin/python

      

        
        
        
      

    

Error_2

Django Version: 1.4.3
Exception Value:    Reverse for 'pages-root' with arguments '()' and keyword arguments '{}' not found
Exception Location: /usr/lib/python2.7/site-packages/cms/models/__init__.py in new_reverse, line 67
Python Executable:  /usr/bin/python
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These are two errors that I run into sometimes and I don't know why they occur because they appear suddenly, may I know how to avoid these errors.

some settings below

MIDDLEWARE_CLASSES = (
    'django.middleware.common.CommonMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
    'cms.middleware.multilingual.MultilingualURLMiddleware', #CMS
    'cms.middleware.page.CurrentPageMiddleware', #CMS
    'cms.middleware.user.CurrentUserMiddleware', #CMS
    'cms.middleware.toolbar.ToolbarMiddleware', #CMS    
)

INSTALLED_APPS = (
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.sites',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'django.contrib.admin',
    'django.contrib.admindocs',
    'cms',
    'mptt',
    'menus',
    'south',
    'sekizai',
    'reversion',
    'cms.plugins.text',
    'filer',
    'easy_thumbnails',
    'cmsplugin_filer_file',
    'cmsplugin_filer_image',
    'cmsplugin_filer_teaser',
    'cms.plugins.link',
    'cms_themes',

)