I read through the thread a few times, and it sounds to me like you have equipment on a UPS that you are trying to control using X10. Either that, or you have X10 equipment that you are trying to control with a controller that is plugged into a UPS - I'm not sure which.
Either way, it makes sens to me that it *would* work with an "offline" UPS, and it *won't* work with an "online" UPS.
The problem is that X10 signals can't pass through the inverter. So, in the "offline" UPS, the load is connected directly to the AC line until power goes out, and it switches to battery. The signals are able to pass through, since there is no inverter in the way.
However, the "online" UPS always has the load running off the battery, via the inverter. That's why it won't pass the X10 signals.
I hope that explains it.