I had this problem before and I can't remember how I fixed it, and it is driving me insane again.
I have the latest QT codec and RedRushes bt when I add my clips in and set the settings I hit Start and it renders 1 of 1 frame for each clip. When you look in the output folder there is nothing in there.
Similarly, if you try to render an ALE, there is nothing in there.
Any help, much appreciated.