I seem to be having problems with my cable release in both manual and auto focus with mirror up.
I have used both the MC-DC2 with the Nikon GPS and the Nikon MC-30. They will almost always make the mirror go up, but the second press does nothing and I have to press the shutter button on the camera to finish the shot.
I bought a new MC-DC2 thinking maybe the wires were pinched or broken, but it did not solve the problem. The other day I switched to the MC-30 and had the same problem.
It does not happen all the time, but when it does happen, it is a problem for maybe 5-10 shots.
I am almost positive the shutter release on the body works fine.