I have noticed that when the ESRI server logic encounters some problems it incorrectly returns HTML instead of JSON. It looks like this is happening to you.
Obviously, what you have to figure out is what the underlying server error is so that you can resolve it.
It would be good to see the full response, i.e. all the HTML, which the server returned. Also, you will want to check the REST log, ArcGIS Server Manager log, and the NT event log.