Both switches control the light. Either can be used to turn the light on and off. One does NOT have to be in the 'on' position for the other to operate.
This is just a typical 3-Way light switch set-up... very common.
From what you describe, it sounds like you have a
phase issue. Your Transceiver is on one house phase and your lamp modules are on the other. Why it works when this light is on is somewhat of a mystery. A light circuit should not bridge electrical phases. With the light off, turn on your dryer (assuming it's a 240VAC and not gas) and see if you can control the lamp modules now. If so, I suggest getting a
phase coupler. If not, maybe move your Transceiver to another outlet.
What is the model number of the Tranceiver? (
TM751 / RR501 / etc.) Are you controlling the lamp modules with a remote? I'm sure you already made sure the outlet the Transceiver is plugged into does not have it's power removed when the light is turned off.
Also, it appears from what you said that your PC is on the same electrical circuit as the Transceiver. PC's are big
noise generators and can cause X10 problems as well. There are also
filters available for keeping noise off your electric lines.