BuddyPress.org

sorry, it is WPMU 2.9.1 and Buddypress 1.2.3 (I think)
there are many plugins installed and some small modifications, but not related to PM

what I think may have triggered it, is i had to delete some spammers, and deleting via buddypress did not seem to work, so I went to wp-admin and deleted one or two spammers there,

but it asked me to transfer their posts to my (admin) profile,
there were no posts, but that dialog box came up, so I accepted it.
and it had completed the delete process successfully,