You must change the value 'web.base.url' to 'https://' under settings -> parameters -> system parameters.

I have tried setting up Odoo 8.0 with a reverse proxy (NGINX) with SSL (443) but without success. Odoo always rewrites the URL to "http://" instead of "https://". Does anyone have an idea?

When the user logs being superuser, parameter "web.base.url" is update.
If you do not want to update. Add to the model "ir.config_parameter" record with key "web.base.url.freeze".
Since then the system retrieves only parameter defined by us "web.base.url" and does not update it.

This works with Odoo 8.0

you must tell the server, that you are using https (proxy_set_header) - a complete example for a https-Reverse-Proxy could look like this:

    upstream odooserver {
        server 127.0.0.1:8069 weight=1 fail_timeout=1800s;
    }

    server {
        listen 80;
        server_name you-domain;
        return 301 https://your-domain$request_uri;
    }

    location / {
            proxy_pass http://odooserver;
            # force timeouts if the backend dies
            proxy_next_upstream error timeout invalid_header http_500 http_502 http_503;

            # set headers
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forward-For $proxy_add_x_forwarded_for;

            # Let the Odoo web service know that we're using HTTPS, otherwise
            # it will generate URL using http:// and not https://
            proxy_set_header X-Forwarded-Proto https;

            # by default, do not forward anything
            proxy_redirect off;
        }

Has anyone tried a reverse proxy on OpenERP 8?