I keep having the same error myself. But I don't believe it's specific to this buildOS since I spent a day getting a rom to load from Verizon guy's OS builder too. (I only had trouble with his RC1, not with his 0.6c, but I didn't do it enough with the earlier version to know whether that was a luck of the draw issue.)
So, specs, I have a Dell E1705 laptop running Vista 32-bit, and my phone has bootloader 1.00. Oddly if I try to run the RUU with the phone connected to either of the side-jacks, once it gets to the part in the bootloader, it crashes the program. If I run attached to one of the 4 ports on the back of the computer, starting while not in bootloader, it completes the initial phase, but then has a connection error when the phone reconnects itself through bootloader mode. (I also tried multiple cables, killing all other programs, creating a new, clean Vista user, etc., to no avail.)
After 12 hours of trying, I finally got lucky yesterday by physically disconnecting and then reconnecting the phone toward the beginning of the bootloader phase with Verizonguy's RC1. Today I finally got it to work with Helmi's new WM6 kitchen by starting in bootloader and running the RUU with full administrator privileges. I should also note that I had to run the kitchen itself with full administrator privileges to get that to work too.
I doubt this is related to either kitchen, and more likely related to some Vista-USB mishegoss. I considered flashing with Verizon's stock ROM, but the only one I have is the new 3.3 and didn't want to have to deal with getting my bootloader down from 2.05 to something workable.
One other thing -- in between my first easily successful trial with Verizonguy's 0.6c (which, again, I only ran once so it's entirely possible I just got lucky and the problem was there all along) and my interminable bout with trying to install from his RC1, I found and installed a USB update off of Windows update:
941600 I have since uninstalled that update (as well as the two others I put in at the same time) but it doesn't seem to have helped either.
If anybody has any ideas how to more reliably update through Vista or what I might be doing wrong, I'm certainly all ears -- especially since I just noticed my newly flashed phone seems to now be booting only to a white screen as I've typed thisindicating I probably need to reflash . . .
Thanks also to everybody for what must be incredibly time consuming work. I look forward to playing with my phone as a distraction from the work I ought to be doing.