I have almost the same setup - a Palm Vx, and a Serial->USB converter, thoug this is from Palm.
I have checked my settings - they match yours. The only thing I can think of, is if you have selected to enable hotsync software at system startup. But then again, if it works with your Treo, then it should be ok.
What happends if you use the cradle and presses that hotsync button - or use the button at your Palm? (In the 'HotSync' program)
Jens Peter