What I think you are saying is that accessing calibre's content server from your device works if you are using the web browser connecting to
http://IP:PORT/. However, it sometimes doesn't work if you enter IP and PORT into CC's settings on the same device. If this is true then it really sounds like there is some "security software" on the machine running calibre that is getting in the way. Of course the suspect is Norton.
When you open
http://IP:PORT/ in a browser you get calibre's home page. However, CC will try to open
http://IP:PORT/opds to get the application interface. If the browser works but CC doesn't then something on the calibre machine is blocking access to /opds.
I might be able to tell you more about what CC thinks is happening if you send me a CC debug log as described in
http://cc_faq.multipie.co.uk/index.php?solution_id=1068. Please erase the multipie.co.uk address.