VFD (?) tripping RCD

[mocha]

Assuming you haven't overlooked anything or missed something in the tests you have already done... It sounds similar to a problem I had with radios some years ago. ...

Bring some power from the house or from next door! but bypassing the RCD unit, with a standalone RCD if necessary, (but if you are happy to have a hand in your pocket while disconnecting earths you probably wont want to! lol)

Try plugging each of the suspect items; the VFD? into the extension lead with everything else plugged into the main RCD, as soon as it stops dying or the problem disappears you have the suspect device cornered for further investigation

or... find a way of removing DC from the AC line back to the RCD, or split board supply??

The RCCD you point to; 40amp? is it 30 or 100 mA?