Yes, it sounds like the user has sent 21 incorrect commands to LISTSERV
in a row and has been served out.  Experience tells me that this may
be because she has a mail client that sends back an acknowledgement when
she has read the mail, and that the acknowledgement is going back to
LISTSERV, which is treating it as a command error.  You can restore service
to this user by sending
 
SERVE [log in to unmask]
 
to LISTSERV.  (be sure you put her appropriate userid in:)
 
Note that WHENEVER this happens to a user, they get a message from LISTSERV
as to what has happened and exactly what needs to be done to restore
service.  Any user (other than the one served out) can restore service
to the user with the SERVE command unless the user has been "administratively"
served out.  I doubt that has happened in this case.
 
Nathan