title: How do you fix a Trac installation that begins giving errors relating to PYTHON_EGG_CACHE?
body: <p>We've been using Trac for task/defect tracking and things were going well enough, but this morning it started serving up a 500 error. Looking in the Apache error_log, I get a stack trace that culminates in:</p>
  
  <pre>
  PythonHandler trac.web.modpython_frontend:
    ExtractionError: Can't extract file(s) to egg cache
  
  The following error occurred while trying to extract file(s) to the Python egg
  cache:
  
    [Errno 13] Permission denied: '/.python-eggs'
  
  The Python egg cache directory is currently set to:
  
    /.python-eggs
  
  Perhaps your account does not have write access to this directory?  You can
  change the cache directory by setting the PYTHON_EGG_CACHE environment
  variable to point to an accessible directory
  </pre>
  
  <p>So I explicitly set PYTHON_EGG_CACHE to /srv/trac/plugin-cache. I restarted Apache. Yet I get the same error (it still says "egg cache directory current set to: \n\n /.python_eggs.")</p>
  
  <p>How should I proceed? Is the simplest thing to do to reinstall Trac? If I go that route, what steps do I need to take to ensure that I don't lose existing data?</p>