Since it's a 'new' d/l, I'd call the 800 nbr and grill the Xbox folks about it.
You may have to do a system reset that removes all previous updates to get it working, but you'll then have to re-download all your other patches and some of your d/l content to get them working again.
Im having this same exact problem, except im using my original disc. I deleted system data for this game and still nothing. I can get on coop no problem but versus is still giving me that lame message. Whats with this guys?
On a side note, if you have a newer version of the game, you can tell by the little round piece that holds the disc: if it's solid it's newer and if it has slits then it's an older one, it has a better chance of working. A used copy (maybe even a new) is worth about 4 bucks last time I checked.
Give it a shot and reply if it worked or didn't so I could probably help you more.