Nginx reverse proxy not working with IPv6

Hi there! I wanted to test an Nginx reverse proxy configuration for Ajenti with IPv6, so I took the configuration from the documentation and changed it to IPv6 only:

server {
    server_name localhost;
    client_max_body_size 20m;

    listen [::]:80 ipv6only=on;

    location /ajenti {
        rewrite (/ajenti)$ / break;
        rewrite /ajenti/(.*) /$1 break;

        proxy_pass https://127.0.0.1:8000;
        proxy_redirect / /ajenti/;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

Now I went to http://[fe80::20c:29ff:fe88:13a5]/ajenti/ (that is the IPv6 address of the virtual machine I tested this configuration in), entered root/admin and ... well ... I only get spinning wheels. I waited a couple of minutes before I gave up. The Firefox developer console says:

NS_ERROR_MALFORMED_URI:  resources.js:30

Unfortunately, resources.js consists of minified JavaScript code, so I could not debug this problem (or the allegedly malformed URL) any further.

Any suggestions? Unfortunately, I tried this because I wanted to retire all of my IPv4 services, so I hope that reverse proxying Ajenti with IPv6 is possible.