The bug originated from players who hacked their own games and then played multiplayer games online with others. It only affected players on Xbox 360; players on PC and PS3 did not face an issue.
Today, the patch became available on Xbox Live. The description reads, simply, “Added security to prevent data loss tied to malicious violations of the Xbox Live Code of Conduct through an unsupported third-party program.”
Xbox 360 Update Information [Gearbox Official Site]