Since you're running with a Windows machine, perhaps my comments don't apply, but...

I use Pronterface on OS/X, and I have a RUMBA. Also like you, Arduino worked fine going from Mac to RUMBA. And like you..., I couldn't get Pronterface to talk to the RUMBA. Baud settings on the Mac side made no difference.

So..., I set the baud on the firmware side, in Configuration.h:

#define BAUDRATE 115200

re-installed the firmware, and Pronterface has worked ever since.

HTH.