Issue
An error message indicating that an invalid content type 'text/html' rather than 'text/xml' is being returned. This is due to a bug in your web server.
Resolution
If you have the option to use the Tomcat port directly (rather than going via a web server), this should solve the issue. Otherwise ...
Confluence Hosted / Resin Web Server
If using Confluence Hosted by Contegix, you simply need ask them to switch from mod_caucho to mod_proxy:
Apache Web Server
There was recently a bug in mod_jk - see this thread: http://www.nabble.com/mod_jk---answers-from-appsrv-often-changes-'content-type'-from-text-xml-to-text-plain-t4736483.html for details, and a link to download the fixed module.