By the debug print you sent me, it looks like you start Yawcam automatically hidden to the traybar.
Are there any difference if you don't start in hidden mode at startup?
When you get a black preview window... What happens if you close it, disable all output types, wait 6 seconds, and then open the preview window again?
Is it still black?
Maybe it´s a kind of time-out problem with initialising the cam-driver the first time?
Hmm... Maybe. The first time when Yawcam starts slowly, do you see the intro image during the whole time? or does it disappear as soon as in the second time you start Yawcam?