I have just recovered from the above error by following the TID here
http://support.novell.com/cgi-bin/se...?/10074727.htm and am
looking for possible reasons as to why it happened as it has cost me half a
day to put everything back in place.

This error occurred on Friday when we tried to add a user to one of our
access rules. Other than adding/removing users on a fairly frequent basis
our BM config is fairly static and has been running without errors for
several months. An NDS health check shows no problems and apart from BM we
only run iFolder on this server. We are using NW6SP5 with BM37SP3.

Any suggestions as to how to prevent this in the future?

Thanks