My partner just created Yet Another listserv from user request, and when she
did that, the drop-down menu of lists in the web interface list management
page disappeared.  We deleted it and all was well again.  With some
experimentation, we discovered it wasn't that particular list, just any one
that put us over 53 (nice round number, eh?)

Does anyone know what is causing this behaviour?  I don't see anything in
listserv.log.