With yesterday's release of the "Sir Hammerlock's Big Game Hunt" expansion for Borderlands 2 came a new compatibility pack to ensure players without the DLC could still co-op with those who bought the new content. But with that compatibility pack on Xbox 360 came new issues, including one that prevented players from downloading it and thus from playing online on Xbox Live.
Gearbox wrote on its support site that they're working with Microsoft on fixing the issue for good, but in the meantime users can log into their Xbox Live accounts through their browsers and manually add the compatibility pack to their download queues here.
If that doesn't work, they suggest you open a support ticket.
"Sir Hammerlock's Big Game Hunt" is our favorite Borderlands 2 expansion yet; what's yours?