I'm on a development system (thankfully) and just had the define fields window open for ~ 30 minutes. No other client using the database. After closing the window and saving the changes (appears to go ok) FMP got stuck on a sorting records dialog. After 10 or so minutes I force-quit FMP. Opening the database again causes a beach ball and no progress.
The FMS activity log records that "Could not pause database "AdvBuildingQLD" because of lock conflict(s) with 1 database user(s): admin (vaughan.bromfield)". I assume this is a backup process complaining. I have progressive backups running, plus hourly scheduled backups.