Nope, this is a development install, running locally. All GET requests do this, POST requests go through fine. No plugins whatsoever, the error comes from within the middleware.
I analyzed the standard install in much more detail and found that the docker image launcher uses bundle exec config/unicorn_launcher -E production -c config/unicorn.conf.rb to launch unicorn. I used RAILS_ENV=production bin/unicorn instead. I guess I missed the run configuration. Problem solved.