I had an interesting finding... The story in short: My first FF400 had some hardware trouble so I returned it, and got a replacement. The new one was ok. The issue I had with the rotary encoder is away. So far so good.
I connected the new FF400 to the fresh out-of-the-box MBP latest and greatest version that I received in parallel, and of course no chance to get rid of the red light. 400to400, 800to400 cable --- no chance
I used my Lacie D2 Quadra inbetween, and ran the HD in OFF mode. Interestingly with the first FF400 it worked seamlessly. The new FF400 however has shown the red light all the time. I connected the FF400 Power Supply and same issue. I could resolve it by switching the Lacie HD to "ON". So the HD runs all the time and produces fan noise, but fortunately the FF400 was connected to the host system. Again, this issue was new to me. The first FF400 worked even with the Lacie in STDBY mode very well together with my iMAC I own as well. The new FF400 does not.
Conclusion: It looks like even the FF400s have variances in the Firewire physical layer. From my technical perspective it must be something in the voltage level, which can't be interpreted well by the FF400. Maybe the Macs "fire" too low voltage... I don't have the time to investigate into the electronical details. I can not live with an Firewire audio interface, which requires a running Harddisk inbetween during audio recording...
I returned the new FF400 and will go with another brand.
It's a pitty...but true... Buy buy RME
P.S.: RME, get this Firewire issue fixed, and you are the winners!