When updating my operating system from OS X 10.9 to 10.10 not only my Keystation 49e stopped working but also my Audio interface Mbox2. I searched the Avid forums and homesite for a solution and finally one dude at the forums helped me out. Avid has through Facebook finally announced that Mbox2 are now legacy hardware and no new driver will be compiled – sad. Its an expensive piece of hardware.
The solution that worked for me is to downgrade the driver. First uninstall, reboot then install an earlier version. Unfortunately one has to reconnect the USB-cable after booting up. It seem like OS X can’t identify the interface until that point. I can live with that.