The problem in my previous post about the FD10 coupler/filter has been resolved. They seem to work as expected. The real problem was a loose cable elsewhere.
I have one issue remaining. I'm unable to send commands to a certain switch if the lights are on! These lights are LED lightbulbs, so I assume that they disturb the X10 signal. I know that X10 is a 120KHz signal on top of the regular sinus (in my case 230V 50Hz), but what's the amplitude of the X10 signal?