This is likely due to a firewall issue on your blog’s server that is blocking requests from the Written.com API. All of the API requests out platform make to your blog come from this IP address: 184.108.40.206 *
*Using the using the 'Bad Behaviour' plug-in? Make sure you also whitelist the URL fragment : /written_api
This is likely due to an issue with mod_security on the server where your site is hosted. To fix this, please contact your web host and tell them to check the web server logs for mod_security triggers relating to your blog’s domain. Your host should see an error triggered in these log files, and then they can apply to appropriate fix.
After any fix, you should always hit the "Resync with Written" button to reconnect Written with your plugin.