Schedule Manager and "Error validating user X while processing calendar appointment"

I have made a custom database starting with a clone of resource.nsf. Have unhooked all references to busytime.nsf (clubusy.nsf) as we do not need the busytime connection. Purpose of database is a master shared calendar, but NOT linked to users personal calendars. The rooms, etc., do NOT have corresponding entries in Names.nsf. This has all been disabled. Problem: Notes server log is filling up with entries “Error validating user X while processing calendar appointment (NoteID:NT0000xxxxx) in database calendar.nsf: Cannnot find user in Domino Directory”.

It looks like this message is coming from the Schedule Manager when it runs. Where does Schedule Manager know about this custom “calendar.nsf” database? It should only run against “resource.nsf” and the mailfiles?

TIA.

Randy