It is possible to enable the new service "Webserver" and bind TCP port 80 for it, even if FreeNAS WebGUI already use TCP port 80.
The result is that you've your WebGUI for FreeNAS is gone and you are smoked. You can possible restore or change that port in the config.xml file. But in the image version that file is read-only.
I had to reflash my USB-stick to be back in business... :-(
It should not be possible to use the TCP port 80 for the web server, unless you already have changed the default TCP port for the WebGUI.