Machine trips RCD when turning on

I've used my Breville (US) Barista Express successfully in the UK using a transformer. I've just moved houses, and now every time I try to turn on the machine, it trips my RCD. How can I diagnose/fix this?

Couple of observations:
- The transformer works, it turns on/off without problem when nothing is plugged in, and also powers a Breville toaster oven just fine
- If the transformer is in its "off" position (but the main switch is on), the RCD trips from just plugging the machine into the transformer
- If it's in the "on" position, the RCD trips from trying to turn on the machine. For a brief moment initially, the machine turned on this way but tripped the RCD when trying to brew, and now it trips even when turning on


Any help appreciated to diagnose this!