This error is likely due to Splunk being unable to talk to itself. It's a known issue with Windows, see:
http://docs.splunk.com/Documentation/Splunk/latest/ReleaseNotes/WorkaroundfornetworkaccessibilityissuesonSplunkWindowssystems
This answer has the most detailed information, in terms of users' experience.
http://answers.splunk.com/answers/68368/splunk-web-throws-responsenotready-error
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