I had this on a site a couple of years ago. I think it is service pack/EME and PV combination specific, but that isn't much help if you are on a demo version!
A workround that usually did the trick was to delete the BW login that you are having problems with and then insert it again. It may sound silly, but it did work until the problem went away with a more recent editor and service pack.
The log file, by the way, is a distraction - the error message is inappropriate to the problem.
Hope that still works