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The high dv/dt of the inrush could cause emission through stray capacitance and to earth like that.

Or just the general "electromagnetic loudness" of inrush could have gotten coupled into the mains wiring and then noise_bombed the electronics in the RCD....making it trip spuriously.

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So you are saying you have significant 50Hz ripple on the VFD input..?

This 35v  presumably comes from that switch mode transformer with the yellow tape round it.

So now i am wondering if it is overheating and has insulation damage inside it...

Taking it out to hipot test it doesnt sound like a great idea though.

I suppose the other way would be to get a separate isolated 35V supply and solder wire that in after the yellow transformer...having disconnected  the yellow transformer from its ouput ...eg desolder the diodes what have you.

It does look a very small transformer for the power you mention. I dont see a PFC inductor either on the board you show.