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Maybe the RCD is getting corrupted by the inrush, which is a "Loud" electromagnetic event, and happens when it is powered up.

So i am thinking. ..can you have the VFD on no load, and disconnected from motor, and then power it up without the inrush relays in circuit. Or maybe you can set up an external inrush resistor.......then the shorting relays wont be able to short that out.

Wear eye protection as inrush resistors can blow up.

300+nF sounds very unusually high indeed for a y capacitor on the mains....but you are saying the tripping happens when the motor isnt even connected.