Thanks a lot for your reply! I’ve finally solved this, the reason was that in my config/prod.exs there was a setting
config :wind_world_web, WindWorldWeb.Endpoint,
force_ssl: [hsts: true]
which probably assumes that there are paths to certificates in the config file present which is not the case with letsencrypt used by fly. So I added infinite redirect loop logic in two places with both force_ssl and my own code, that’s why commenting my code didn’t yield any effect.