You need to delete the Agfa firewire kernel extension named "AgfaScannerFireWire.kext" or for Umax "UmaxFireWire.kext".
The is most easily done via the terminal application.
The filename of the Agfa firewire kernel extension installed by FotoLook might also be "AgfaFireWireSBP2Scanner.kext".
Where to find the "Terminal" application?
You will find the terminal within /Applications/Utilities/.
ls /system/library/extensions you will see if an "Agfa(whatever).kext" is present in your system blocking SilverFast's access to your Agfa scanner.
Please note the sorting: alphabetical, capital lettes first, so "APPLE" is listed above "Agfa".
sudo rm -rf /system/library/extensions/Agfa* will let you get rid off the kernel extension, letting you use SilverFast again (reboot is necessary).
You will need the system administrator's password (if set) to remove the kernel extension.
However, you will not be able to use FotoLook software afterwards, as it needs the Kernel extension for scanner access.
But provided with the FotoLook tools there is a helper app which does install the kernel extension with a simple double-click, so it can easily be restored.
BTW, the command
kextstat | grep -v appleshows all loaded kernel extensions that were not provided by Apple (with the Mac OS X).
The '|' character is entered by holding the option (or: alt) key and pressing the '7' key.