Have you enabled security? If so, the browser does not prompt for the username and password. You need to put them in the URL:
http://username
assword@nnn.nnn.nnn.nnn/mobile
But, I was reminded of the need for the username/password after looking at the log files for this as it wasn't working when I just entered the ip address.
The browser doesn't update the to show anything if there is no response from the server. It isn't a great browser. But, if you are not seeing anything in the calibre server logs, then the browser isn't seeing the server. Can you ping the address of the device from the machine running calibre?