Hi,
Exact same problem here.
My Impulse account details are apparently "Invalid" when I try and log into multiplayer.
My friend, after buying it for himself, bought it for me buy logging into my impulse account on his computer and buying it again. Then, after having bought it, he logged back in with his own account to play. I logged in on my computer, registered it using his email adress and the key sent to his address, but now we're stuck........