It sounds like there is some kind of isolation issue with the drive speed control.

If the drive speed control is being connected to ground, then that can cause an RCD to trip.
The speed control circuit on these kind of drives sits around half of mains voltage, so if you connect it to 0V/GND (the control electronics will typically tie 0V to GND), then you've just created an earth leakage which an RCD will detect, but as it'll be pretty low current, it may not always be enough current to trip the RCD.

I wonder if the isolation on the controller has failed.
A manual fix, would be to install a potentiometer to control the spindle speed. There is a slim possibility that I may still have a suitable potentiometer kicking around the workshop, but I won't have any time to look until next week, as I'm working at an event all weekend.