Both games were released today, although neither title can be found by directly searching their names. Users who try download said content or access either game from the dashboard will receive an error message. However, both games can still be purchased via the Xbox Live website and downloaded remotely.
Microsoft is working on addressing the problem, according to a recent tweet from Sega producer Stephen Frost.
"For those trying to download Nights or Sonic Adventure 2 on XBLA, we are aware of the problem," said Frost. "Microsoft is looking into the issue now."