The OVA Image doesn't seem to be IPv6 ready.
Nginx isn't listening on IPv6. I added
listen [::]:80;
to the config but still. When I access the WebGUI it somehow tries to access the IPv4 address which in my case isn't routable.
Only when I set something like this I'm able to connect to the GUI:
http_bind_address = [2a00:fea:3a:3b::5]:9000