Today I loaded a saved project and had no sound out of the master. I could see audio entering it (with a plugin inserted) though. I checked F4 and found that I could not assigned a hardware out to the stereo out nor could I click the little box with the check in it to unfold the left and right channels.
In the end I just deleted the bus and created a new one, then reassigned all my channels. Just curious if this is a new bug as never encountered it before.