It has (again) to do with my automatic backup of files (see: Error since files resides on Dropbox). I am modifying something in Manager, but the backup is still in progress and this temporarily flags the *.manager file as being read-only, so Manager itself gives an error:
SQLiteException: ReadOnly
So either Manager itself should mark the *.manager file as read-only when doing modifications, or I must exclude the Manager files from being back upped.