As explained in the other thread, if you have ANY file which is either not the original one from 9.0 OR it's already updated to 9.1, the updater would not work.
To apply the update correctly, you first need to restore all your DLL files (those in the MODULES folder) AND the FS9.EXE from the original 9.0 CD, delete the BACKUP folder, AND run the 9.1 updater.