Both sides.

The script you are using to fetch data from the server expects JSON back, and croaks if it does not get it.
That part could be improved with Error/Warning detection and handling.

The server side code that runs on the hosting service is what provides a response to a request from your clinet script.
That should also behave itself when JSON is requested, and provide a JSON-only response.
Apparently, it is not doing so in this case - it is sending an HTML style warning message.
You can blame (lack of) documentation for this, and/or poor design/coding/testing.