Hi all: Many thanks for all of you who tried to help and thus helped!
I decided to check, whether dhe FW16E Externstioncard on the Motif XF was working correctly. I did this by bringing my old Win10 Machine up again and check. The Connection worked when I re-plugged the Firewirecable a couple of times. So I was sure now that the FW16E card works properly.
Then I reconnected everything to my new Machine with the Win11 installation and … voila… now all drivers are installed correctly and things work as they should. I am not really sure what brought it to life… maybe something was just wrong with the physical connection (Plug).
The good news are: 1) No need to use legacy Firewire Drivers on Win11. 2) Both, VIA chip based FW card and TI chip based FW card are working correctly with the respective Win11 drivers!
What IS necessary is to disable kernel-Protection in Windows security.
Great that it works now and that it shows that all this “old stuff” still works (Old stuff: FW16E in the Motif, Firewire audio interface - Presonus Firepod, with an appropriate Universal Control version that is still available for download).
I am happy 