The number of webmail licenses was exceeded. The user list was reset, and this cleared the problem.
Due to this action, all address books were inadvertently deleted.
Unfortunately the backup failed as well.
This does not affect email data itself, only the webmail address books.
New procedures are being written to document the process of saving the address books before resetting the userlist.
In addition, a more frequent backup plan is being put into place. There will be weekly backups of the address book data.