This did happen more than once, so I think it is a common behavior due to a bug.
On my Mac with OS X 10.6.8, sometimes the UR824 is seen as “stopped” by the music software. The Audio and MIDI Setup application can see it, but it cannot change the sampling frequency from ‘0.0 Hz’. Kontakt 4 sees it as “stopped”. Logic 8 tries to “initialize” Core Audio after choosing the UR824, but it freezes while trying.
Oddily, Audio and MIDI Setup allows me to change the UR’s synch source, so the interface and its driver are not totally crashed.
Console only shows these lines, when searching for Steinberg or Yamaha:
03/03/12 19.21.53 [0x0-0x451451].com.yamaha.dspMixFx_UR824 will set.
03/03/12 19.21.53 [0x0-0x451451].com.yamaha.dspMixFx_UR824 did set.
03/03/12 19.22.24 [0x0-0x453453].com.yamaha.dspMixFx_UR824 Qt: QCursor: Cannot create bitmap cursor; invalid bitmap(s)
There is no apparent way to restart the driver when this happens. The only solution is to restart the Mac - a way of solving a problem that I had not seen for decades on a Mac!
Is there any hint on how to solve this annoying issue?