I had some very bad experiences using active USB extensions a number of years back. All kinds of random electrical noise would cause a hiccup that would momentarily cause the USB connection to reset. Enough of those resets in a short period of time would cause the CM15A to go into an error mode (as confirmed by the developers). Mine would often be triggered by turning on the lights in my den (a room near - but not on the same circuit as - the CM15A and the computer. Sometimes my furnace would cycle on and cause it (again, not on the same circuit or phase as the CM15A and computer). Some people have had really good success using USB extenders, others did not. The nature of the communication between the CM15A and X10Nets service makes them very sensitive to disruptions.
In my case, I tried a number of different extenders, several different CM15A's, and even two other computers. Nothing worked. In the end, I just moved the CM15A close enough to use a "standard" length USB cable, and plugged it into the outlet (on the front of my XTB-IIR) using a long extension cord.
Another thing to check is the power saving settings on your USB ports. Make sure they are not set to allow the computer to turn them off to save power.