I was working in a dev environment and noticed that the “All Users” search scope was no longer appearing in the drop down list.
This perplexed me because I had not changed any search scopes or permissions in a long time. All I had been doing was merrily cleaning up some attributes the schema no longer needed.
Turns out one of the attributes I deleted was referenced in the search scope in the list of attributes to search and the results to display. Removing the now no longer existent attribute allowed the search scope to be displayed again after a service restart.