IS the usb to DMX a box ? or small dongle thingy ? Actually if either is close to the fog machine they are most likely picking up magnetic interference. As a precaution I would do a couple of things.
That's a recipe for disaster - any tech who knows what he's doing shouldn't have set you up like that.
It is good you have fixed the interface problem but you may cause yourself another problem. Unless your dimmer states specificaly that it can handle inductive loads I wouldn't leave the fan on it. Try and find another power point that is not on the same circuit as your laptop. The dimmer may be fine but if the fan plays up I would definetely take it off the dimmer.