Error message: Index is not generated on server

My client and Domino server are R5.x. I have an FT-indexed database updating hourly. I have a Lotusscript button which basically does an FTSearch on the database and puts the documents it found into a private-on-first-use folder. It works just fine when I run it and I have never had any problems until a user attempted to run it and kept getting an Err# 4000 with the message, "Index is not generated on server". I am confused as to why this is happening. Even if the database were not FT-indexed, the db.FTSearch should still work (although less efficiently).
Anytime you open a view or use a search, the Notes server uses a scratch area on the server drive to hold temporary data. This scratch area needs to be larger for someone that has not recently opened a database that contains a large number of documents. If the server does not have enough drive space and error occurs.