HTTPClient POST tries to parse a non-JSON response

You’ve put responseType: 'text' in the wrong section of your httpOptions – It should sit outside of headers, like so:

private httpOptions = {
  headers: new HttpHeaders({
    'Accept': 'text/html, application/xhtml+xml, */*',
    'Content-Type': 'application/x-www-form-urlencoded'
  }),
  responseType: 'text'
};

With what you had before, a request header of responseType was being sent to the server, rather than simply having an instruction to Angular to actually treat the response as text.