That's when you validate the key with AGESettings, right? If this is the case, then you might try to input the key directly into a file so that AGESettings does not trigger. You have to open the game main folder (might be C:\Program Files\Ageod American Civil War) then the folder Settings. There you'll see a file named general.opt, this is a text file, open it with notepad.
At the end of the file, there is an entry like
add your key after the equal sign, save and close. Restart the game.
Hofstadter's Law: "It always takes longer than you expect, even when you take into account Hofstadter's law."