Based on the error message, the problem seems to be on the server itself.
Go into the users' listing and re-establish the preferences for the users.
That may help, and avoid a time-consuming backup and reinstall.
If that does not cure the problem, go back to the most recent backup and
restore that data. There may be a corruption within Aristotle itself. If a
backup is not available, then recreate the Users and Groups files, and stop
short of re-installing Aristotle.
Finally, if the problem still persists, re-install Aristotle and re-create
the users and groups.