This question is answered by

 http://www.lsoft.com/manuals/lsv-faq.stm#5.11

Although the problem covered by the FAQ is slightly different than the
one you are reporting, the resolution is identical--you delete the
listname.ind* files and listname.html file and let LISTSERV rebuild
them (GET and PUT of the list header will do this, or if you have
many lists with this problem, just stop and restart LISTSERV after
deleting the files noted above).

FWIW I'm getting ready to update this particular FAQ answer since I notice
it doesn't specifically cover this problem.

Nathan