Error full text indexing MyDB.nsf: Database is currently being indexed by another process

We have a large Database (750MB) with full text index (720MB) set to update immediately. Because we got the error ‘Updating took over 30 minutes’ I added the configuration setting: UPDATERS = 3 (we have a 4 CPU machine). Since then we got the error message ‘Error full text indexing MyDB.nsf Database is currently being indexed by another process’. Why can I choose more than one update tasks when it produces an error (one task is locking the other) ??? This smells like a bug.