If you look at your the Chrome developer tool (F12), under the Network tab, find the call that returns an error (e.g. 404 or 500, etc.). Look at the Response, it should be JSON data. If it's not JSON data, that means your IIS server has replaced it with its own error page (usually HTML). One way this can happen is if your IIS web.config specifies custom errors using the httpErrors section will prevent JSON from returning correctly. In this case you want to remove the custom errors section in your web.config and retry.
[httpErrors errorMode="DetailedLocalOnly"]...[/httpErrors] (*Square bracket is angled bracket)
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Let us know how it goes. We'll put something in the FAQ to help others with the same problem.