Can't login via Splunk Javascript API

I have a local install of Splunk with default login details (username: admin, password: changeme) and I'm trying to run the examples provided by the Javascript SDK, and I get a 500 Internal Server Error upon login. I am sending a POST request to http:// localhost:6969 /proxy/services/auth/login?output_mode=json and I get this as a response -

{"d":{"__messages":[{"type":"ERROR","text":"Proxy Error","code":"PROXY"}]}}

Both the Node server and Splunk are running on a Xubuntu VM, if that matters. What am I doing wrong?