It works! 
Finally, I am soo relieved.
 
I started a new track circle to test the SSS, as you recommended. They worked (as expected), I could change the speed with the pots.
Then I put everything back together again and suddenly it works. Don't ask me, I haven't changed anything! 
 
Anyway, it's the result, what counts. Right now i am very happy - and can work on the next steps.   
 
Thank you so much for your help, I could have never done it without you. I hope the xmas guy will bring you tons of presents this year, you deserve them all!
 
 
I know, I tortured your long enough and abused your kindness over limit, but please help me one final time with two more question:
 
1. What's the difference between the connections a, b and c? I can't find any explanation in the manual.
 
2. Next step is to include a station model STP. At page 12 of document one ("components"), the wiring diagram is shown in the lower half.
The minus pole from the train trafo seems to be connected to the plus pole of the tracks out of the STP block (in the upper part of the drawing) - that's just a mistake?
 
Ok, that's it from my side - I can't thank you enough for your great help.
 
Have a great week,
Frank