Contact the company - if you're 100% sure that the copy of the game you have is legal. If you've ever loaned the game out to anyone else and they've used it online, you won't be able to go online with it yourself (I presume). I can only imagine that the server would recognise that two identical numbers were running on it at the same time and throw a fit.

If it is perfectly legal, then there's obviously been some sort of mix-up. Like I said, ring the helpline number or something, and you should be able to get it sorted out.