Using 2011.5 with Win 7 and an irritating problem occurs occasionaly.
I am using a SD50 sound module as a midi interface.
For no apparent reason the midi input and output stop working in the middle of a session (not while playing). All I have to do is open the "midi/audio driver setup" window, click OK and the drivers work again. The correct drivers are already indicated when I open the window, I just click OK - nothing else.
So far I haven't been able to see a pattern when this happens.