Why? Well, you've added one line to the barbarian level code, so maybe the game ignores the first line of the diplomacy options, which is always at the top.
How do you fix it? I dunno. Check to see that your game text files still have appropriate lines. For instance, not only in rules.txt, but check to see what the next item after barbarian level is, in game.txt or labels.txt. Sometimes tinkering with these cause the game to miss the intended popup text, resulting in such messages as "Sewer System attack near Valladolid!" Let us know if you are able to smooth it out...