michrech: I see you have your UPS power failure light solved. (I'm only quoting the relevant parts.)
I have a normal, 2 pin, lamp module ... that I plug into the back of an APCC UPS ... I use the "Slim Switch" and a wireless transceiver ... The lamp has a 12w CF bulb .... As soon as the power goes out and the UPS switches to battery, the lamp module automatically turns on (I don't know why, but it is perfect for what I need).
Using a CFL lamp with an X10 lamp module [LM465 (?)] is not advised and even cautioned against by the X10 manual.
"
The Lamp Module can be used to control an incandescent lamp rated up to
300W. It is not suitable for other types of lamps such as fluorescent or energy
saving lamps, low voltage lamps, or lamps that include a dimmer control.
Caution: Do not connect an appliance such as a coffee pot or heater to a Lamp
Module. It may damage the Module and/or the appliance and could cause a fire
hazard. Use an Appliance Module instead."
You might want to be careful if you ever have to change it and end up with a lamp module that (undocumented) has soft start.The CFL lamp may not appreciate it.
Nonetheless, using an appliance (CFL light) with a dimmable lamp module if not recommended; be careful, because if may still provide you with the light you want, but I strongly recommend having an extinguisher handy.
You previously mentioned that you had problems with a transceiver plugged into the output of the UPS, I'm surprised the lamp module is not acting similar.
You also mention using a transceiver and a "Slim Switch"... are these used in conjunction with your UPS powered lamp module? Is so, where is the transceiver plugged into?