Based on this error message, it looks like your application isn't correctly parson a JSON object that's being returned. The first thing to check here would be to verify the incoming JSON payload to ensure that it's fully formed. Have you tried running this through a Lint tool such as https://jsonlint.com/ ?
The fix is to figure out what's wrong with the service and/or the request you're making to the service.
The error is telling you that the service is literally responding with "Error 404" which is probably the source of a human-readable page. The response data from the service is not a JSON payload, which is why the mapping failed.
HTTP error 404 means "not found," so it's quite possible that your request was invalid.