Unable to log in to WPMU DEV Dashboard

I am trying to use WPMU DEV Dashboard and am having trouble. I don't know if it's installed correctly as it doesn't ship with install docs; I extracted it into mu-plugins and added a loader for it, but I didn't do anything beyond that.

When I try to log in to the dashboard it produces an error message: "Your server had a problem connecting to WPMU DEV: "API returned error"".

CURL is enabled and working fine. I stepped through the code and saw that the response from the server has a status code of 200, but the response body is simply "error".

What is the issue and how can I resolve it?

Thank you