UI never finishes loading

The REST API works fine, and I can see the data from the quickstart script but when I go to the UI, I only get the login screen, followed by the two-tone background and a "loading circle" which never completes loading. There are no errors in application.log.

Note: I am running inside a docker container but port 21000 is exposed to the host. I am querying the API from my browser on the host machine, so not from inside the docker container. It works with both localhost and the internal docker ip.

Any help debugging/fixing this would be greatly appreciated.

EDIT: Running the master branch outside of docker, "mylaptopname:21000" works just fine but "localhost:21000" suffers from the same issue.