I've purchased a copy of NVN2 and I wanted to play LAN with a friend, but even after typing correctly my serial keys, I get the message "your cd key is unauthorized". Any solution for this issue?

I had this issue and resolved it by swapping the two expansion keys around. There was an issue where the keys were printed in the wrong order for the expansions and it seems I had that issue. As soon as I swapped the keys the game worked in MP without issue.