
The issue seems to center around Ubisoft's Uplay service, which refuses to accept the download codes issued to customers after they buy the game. Many Steam and Ubisoft customers are complaining that Heroes of Might & Magic 6 has become unplayable since the release of its recent expansion, Shades of Darkness, and the 2.1 patch that accompanied it. Ubisoft is preforming maintenance today to reauthorize players and "gradually set everything back to normal." The post lists possible solutions to some issues, though it seems that those with problems redeeming keys or with inaccessible content will have to wait.

Update: Ubisoft has issued a new response on Facebook, apologizing for the problems users are facing.
