Member subscription with Lyra theme - button does nothing

Hello, I have installed the Lyra theme, uploaded the provided routes.yaml, etc., and a Subscribe link appears. I setup my SMTP server settings in config.production.json, and the e-mail test works fine.

On the Signup page, if I type an e-mail address in the “Type your e-mail…” field, and click the Continue button, the button changes to a swirling animation, and stays that way. Nothing else happens. There are no messages in either the access or error logs, and nothing is output to the console. I have tried with 3 different browsers, and different e-mail addresses.

I am running ghost 3 in a docker container with the ghost:latest image. The docker container is behind a reverse-proxy service (nginx-proxy). I also see nothing in the logs of the http proxy service, which would normally report any incoming connection. This indicates that nothing from my web browser is being sent to the server.