The full error is more like:

The following error occurred while trying to extract file(s) to the Python egg cache:  
[Errno 13] Permission denied: '/var/root/.python-eggs' 
The Python egg cache directory is currently set to:  /var/root/.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. 

 

This happens often if you don't set up a PYTHON_EGG_CACHE environment for your python, setuptools will pick a  place to do it that the zope runner user probably doesn't have permissions to. You can find full info on fixing this here: http://plone.org/documentation/error/permission-denied-python-eggs

 

 

Before you do this recipe of Steve's you'll probably want to use setuptools to uninstall simplejson. Also, you'll need to use the same user Zope runs under to install simplejson once you have implemented Steve's recipe. Also, you may need to uninstall and later (after recipe) reinstall any Python eggs you've put into Plone's python.

To get the changes to take effect, you will need to fully stop and restart zopectl (not just restart the instance).

Note: thanks to cbcunc for the advice on this.