That's very strange, right? I've never seen that error but I can't claim I have that much experience either. I also run Windows under Parallels, but it's Windows 7.
I'm sure you've likely already tried this, but I wait until Windows (under Parallels 7) is fully started and waiting before I connect my Shobot's USB to my MacBook Pro, forcing it to ask me if I want the newly-inserted USB connected to my Mac, or if I want it connected to the "virtual" PC. At this point I tell it Windows. It asks if I want to remember the choice but I decline, as I find I have no problem with Windows recognizing the Shopbot if I do it like this. On times where I've had it connected before I start Windows under Parallels, the Shopbot is sometimes ignored as Parallels seems to assume it's a peripheral outside the Windows shell.
All wild guessing on my part, but maybe the virtual machine is considering the USB port you've picked "bad" when it's really just some software issue with the Shopbot already being connected when you start your virtual Windows machine in Parallels.
Of course, I'm able to do this as I run my Shopbot off of my laptop, which I put away in a drawer if I'm not shopbotting, but if I wanted the convenience of leaving it totally connected all the time I'd probably grow weary of connecting the USB cable all the time. During heavy usage times, once it's running, I leave it connected and running for a day or two at a time.