I know of another person that has had exactly the same trouble (and this week only ). Running Win XP and the previous loader you suggest, there has been no issue with updating MR and MRV Cams in the past, so I am assuming that unless Microsoft have updated the XP using the usual updates, preventing the access to PCMCIA modules that have no authorised signature, then the MRB firmware on the CAM is not upgradeable a second time via a laptop.
Have you access to a MR/MRV CAM that does not have the MRB firmware in it ?