Hello criskolo,
Welcome to the forum.
The behavior you described can be caused by a number of reasons -
- The load on the adjacent X10 switch is generating noise (CFL or electronic ballast) and preventing the XPT communication from being heard by other devices.
- Your X10 transmission levels are marginal, switching the X10 load (incandescent) is sufficient to drag the level below the receive threshold.
- Your "white" wire isn't really a neutral.
I'm a little concerned by the fact that this was once a three way circuit. If you're working on the end of the circuit where power is entering, all may be good. If this switch was the position that supplied the load (likely since you're controlling the load with the X10 switch) it's possible that the white wire isn't a neutral or not the correct neutral.
Please describe the original circuit in detail. It's possible that the power to the two switch positions were actually fed from the opposite phases (220V present across two different black wires). If so, we'll need to determine which phase the "white" wire belongs to.
Boiler
Edit: Just saw Brian's post (which makes a lot of sense). Since this was once a 3-way, there should be another wire available (red?).