An anonymous reader writes: Microsoft has been working hard to win the hearts and minds of developers to support the Windows Store ecosystem. Among the promises: Windows 8.1 update wouldn't break existing Windows 8 apps. Unfortunately, Windows 8.1 has a pretty nasty bug that breaks Multicasting networking for existing Windows 8 apps. It broke apps that do things like uPNP or Bonjour (Zeroconf). Developers are not pleased, and Microsoft has no fix at this time.