Well, after struggling for two weeks I finally got the TMC running!
Downside is that I didn't find out what caused the problem.
I wanted to rule out the DM as the cause. So I installed the TMC, Cubase and the FW driver on a laptop, running on W7/64. With the intention to connect this laptop to a different DM in a studio of a friend nearby, to see if it would work there. However, to my surprise TMC started correctly. So I wrote down all the relevant settings on the laptop.
After that, I re-installed everything on my W7 PC, connected usb and fw, and checked if the settings matched those on the laptop. This time the TMC worked fine.
I haven't found anything that is different from earlier installations. But my impression is that the PC needs to see all connections with all the neccesary settings (like the mackie controls), in order to get the TMC running for the first time. After that first time, the TMC also starts when f.i. the fw is disconnected. It seems to me that the PC remembers the settings after the first time install, but I'm just guessing here.
Most important: back to making music. Thank you all for your help!
Frans