FW:custom_port => hiawatha:443, TLS required, reverse proxy => backend (seahub:8000)
Meanwhile I put a nginx between hiawatha and seahub´s gunicorn.
Setting the mentioned headers in nginx showed no change on seahub´s beahviour.
Workaround is to set
proxy_set_header Host $host:$FW_custom_port;
proxy_redirect http://$host https://$host:$FW_custom_port;