A very important call for the Web UI (the JS exception you provided on 8/26/2013) is failing. Due to this UI is unable to load to completion.
The browser is asking Ambari API for installed services along with status of components. One of those components is Nagios Server. At this point the exception “java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 10″ is thrown.
You mentioned that you did not even install Nagios? Can you please provide the full URL the browser throws exception on?
Also, can you please check if NAGIOS_SERVER is in http://server:8080/api/v1/clusters/staging/services?fields=components/*