Andrea,
I'm not familiar with your machine or the heating element, but generally, here is some info on troubleshooting heating element issues.

With the boiler empty, cool, and the wires running to the heating element terminals off and the machine unplugged, measure with your electrical tester set to ohm or resistance, from one of the element terminals/posts and the other electrical tester wire to the boiler case or body.
What does your meter read? If you can change the scale of the meter, what does it read at each scale?

Is it possible to run the machine with water in the boiler, wires attached to the heating element and wait till it 'pops' your house circuit breaker. Unplug the machine, remove the wires from the heating element and measure again from each one to boiler case?
It would appear from your description that once the heating element is very hot, and the boiler gets pressurized, a ground is created from the heating element to boiler body, creating very low resistance, causing high current flow until your house breaker shuts it off.

If the short to ground is happening right at the point of element thru boiler, I've read of people attempting to apply the electrical insulating varnish to isolate the short. It's electricity..do you want to be 'trying to fix it', or do you want to just fix it and eliminate any future concerns over when it'll fail again?

I guess everyone has a different level of safety tolerance. If a new heating element isn't hard to find, I think I'd be changing that asap. Homeburrero's warning is certainly valid and timely!