I've committed a fix, which is available in acs-mail-lite, 1.0b1, available as soon as the package repository is rebuilt (usually within 24 hours). It is also in CVS, tagged with openacs-5-1-compat, and on HEAD.
Malte, Dave, Jeff, and Bart all helped. Thank you!
https://openacs.org/irc/log/2004-06-10