We're getting emails from people who own Borderlands 2 on Xbox 360, saying that they can't download the new
Mechromancer compatibility update.
Seems that, when prompted to download the new content, they're being greeted with the above message.
My advice? You might have to sit tight. This is the same message you get when trying to access content that's not actually live yet; I'd wager somebody at Microsoft forgot to flip the switch putting the files up for download.
Poor Borderlands 2. It hasn't had the best day for updates.
UPDATE - Turns out this is for the game's compatibility pack, not the Mechromancer DLC.