On Sun, 10 Nov 2019 14:12:40 +0000, Jeff Layman
wrote:

Yes, that could be it. But it would have to be pretty early on in the
circuitry, as it still trips when the on-off push switch is "out". I
guess it could just be a "false" physical break, and it's really an
electronic circuit break which still leaves some connection even after
switching off. It might be obvious when I have a look inside.

Faulty mains filter capacitor allowing small leakage to earth?