I am on my third UM now and it is also sticking in the on position.
How much current are you connecting/interrupting? My guess is it's too much and arcing is damaging the contacts, causing them to stick together. If my guess is right, there's no mod that will make this safe and you need to find another way to accomplish this.
I suppose you could use the UM506 to drive a relay, and use the relay to do the actual switching of the load.
I suppose you could do the same thing with an appliance module (use the switched 110V out of the appliance module to drive a relay. I'm no expert, but I'd guess that there is a way to make a momentary relay, perhaps using a capacitor and some resistors, that would not reset until the 110V is cut, so you could turn the Appliance Module on (which would trigger the momentary relay), and then turn it back off a minute later (or do it faster with a macro), to be ready for the next cycle.