You have to play with your european PSN account if you want to play DLC. If you use your US account, DLC are blocked.

But I have the same error than you this week-end and I don't know why ! The most surprising is that I can play the DLC in single player, but not in multiplayer !!! The message error only appears when I choose a DLC track in multiplayer ! The message appears and appears, I have to quit the game.

I've just reinstalled the pack (Auricom) and the error message has disappeared Try it.