The 18 digit codes don't really do anything other than add the game to their social site and generate you a 20 digit early access code. The 20 digit code is really all that matters. It grants the preorder items and early access.

My advice is if you have a friend you trust who had no issues redeeming their 18 digit key, ask them to redeem yours for you and see if they can get you a good 20 digit key.