SharePoint 2013 - Unexpected response from server. The status code of response is '0'. The status text of response is ''.

I was receiving this error when attempting to use the Content Search WebPart in SharePoint 2013. After pulling plenty of hair out I opened the developer tools in internet explorer (F12) and started a network capture. This showed that it was trying to reach /_vti_bin/client.svc/ProcessQuery which was getting redirected to /_login which was getting redirected to /_windows and finally back to _/vti_bin/client.svc/ProcessQuery. This repeated several times before aborting.

I then tried to reach _vti_bin/Client.svc/lists in the browser which should have returned a list of lists from the web services, but it showed the same in the network capture.

It occurred to me that the web services (client.svc) probably didn’t think I was authenticated and so were maybe redirecting me to login. The login page presumably knew I was already logged in and was redirecting me back again.

In the end I was able to fix this by enabling Anonymous Authentication for the site in IIS Manager. I need to look into the implications of having anonymous authentication enabled, but at least it showed that the problem was due to security settings.